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Gas Furnace Sequence Of Operation

Today I want to give you a basic Sequence Of Operation for a standard gas furnace aka 80% Efficiency furnaces. This will not apply to every single one out there but typically will cover about 90% of what you will find.




  1. The Furnace is in standby mode waiting for a call for heat, the Thermostat has a call for heat, which closes a switch inside the thermostat and sends 24 Volts to the W (Heat)  Terminal on the control board.
  2. The Inducer draft/combustion blower motor then starts.
  3. The Air flow switch will then close and complete a circuit through the safeties as long as everything is okay.
  4. Depending on the furnace either the Hot Surface Igniter (HSI) or Spark Igniter will start. If you have a standing pilot then skip this part.
  5. The Gas Valve will open and the burner will ignite, once the flame sensor senses flame it shuts off the ignition source, if it doesn’t sense a flame it shuts the gas valve off and then tries again usually 3 times before it goes into a lock out.
  6. Next the indoor fan/blower motor will start, some are immediately and others will be delayed by a timer or thermostat.
  7. Once the thermostat is satisfied it will open the switch cutting off the 24 Volt signal to the control board.
  8. The Gas valve will shut off and then either at the same time or a with a delay will shut off the Inducer draft/combustion blower motor
  9. At this point the indoor fan/blower motor may continue to run to get all the heat it can or it might shut off after a short delay.
  10. The furnace is now in standby mode waiting for another call for heat.

That’s basically it, of course there are variations on these, but it should help anyone with understanding what’s going on inside that furnace of yours. I will of course do a series on high efficiency furnaces, but until then if you have any questions let me know.


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  1. Gerry Eisert says:

    On many occasions, my furnace will come on……inducer fan…and will fire, but when the main blower fan comes on, the flame goes out and it reignites seconds later while the main fan is on. It also short cycles once in awhile and it seems to be on a regular 5 minuet cycle, almost timed ! What can be causing this ? Once in full operation the furnace works fine.

    1. justin says:

      It could be your flame sensor is either corroded or needs to be adjusted inside

    2. clinton says:

      do all furnaces have a hot surface ignighter?

      1. Rick says:

        No, some use spark ignitors and older units have pilot lights

  2. Rick says:

    Could be the temperature setting/limit for your fan coming on, it’s getting too hot and tripping on over temp. Once the fan is running then it will maintain the proper temperature in the furnace..

    I would look into the fan cycle control…most of the new ones are a non adjustable bi-metal switch…some older furnaces have adjustable versions…and both frequently fail…hope that helps..sorry for the delay in responding.

  3. Rick says:

    Hi Rick, our gas furnace quit running the other morning. It was bitter cold. I didn’t have a clue on what I was looking for. Anyway, I called a tech and when I come home 3 hours later and turned it on it worked. I cancelled the tech. Someone at work said I could have flooded it. Any thoughts? Thx and enjoy your vids!

    1. Rick says:

      I don’t think you can flood a Natural Gas or Propane Furnace…, even with Propane being heavier than air I’ve never heard nor seen it, the most common problem is the air flow switch. If the line or switch plug up a little bit it could cause the furnace to not come on, sometimes just the vibration is enough to open it up for a little while, causing an intermittent tripping. Another issue could be as simple as a loose connection or a problem with the ignition sequence, if it’s a spark igniter they have intermittent startup issues, HSI – Hot Surface Igniter can have hairline cracks and look good and work off/on…I would check on the air switch. I’ll be posting a video soon on more troubleshooting. Hope that helps you out.

      1. Luciano says:

        Couldn’t it flood if the condensate run back to the vent blower if no condensate line is installed downstream your furnace? The vent blower is low torque and wouldn’t be able to start if there was water at the bottom thus the vent pressure switch not being able to close.

  4. Hi nice post this is completely useful for every one thank you.

  5. Calvin says:

    Hi Rick, that was a well done video. Thanks.

    I have a question regarding my own Gas Furnace. When my thermostat calls for heat I hear it click over and send the signal. The furnace itself was working two days ago but today it refused to fire up. It does not sit it a standing pilot instead it has auto ignition. The thermostat itself has new batteries and the contacts are all free of any dust. Could this be a thermocouple issue or igniter issue?

    Any help you could offer would be fantastic.

    Thanks Rick


    1. Rick says:

      Most likely not your thermostat as long as it’s calling for heat then it’s probably something else, I would check to see if you have any diagnostic lights blinking on your control board, first don’t shut off the power as that could reset any trouble lights. check to see if you have any blinking lights by looking through a small 1/2″-3/4″ hole in the front panel, if you don’t have this access hole then open the front cover and look, usually there is a diagnostic light chart letting you know 2 blinks means X, 3 blinks means X etc.

      If that doesn’t get you anywhere. Reset the power and keep the front panel off and check to see where it shuts down at, should be about the same as my furnace. At least that will give you some check to see if the Combustion motor is running and how far in the sequence it gets, could just be the ignitor as you said, that is very common along with the air switch getting plugged up…always worth tapping on that and see if that does anything.

      Good luck

  6. John says:

    Hi Rick, wondering if you could give this a thought.
    My furnace runs well and I keep it clean.
    5 mins after every full cycle (as you thoroughly describe on this page) my furnace will start-up begin to heat and shut down.
    The other day, immediately following a perfect full cycle, I completely removed the thermostat. –as I expected, after 5 minutes, it started up, began to heat and shut down. At that point nothing happened for over an hour until I reconnected the thermostat.

    Does this sound like I need to replace the main module? The Honeywell S8610U module does not have any DIP, pin, or other manual settings.

    Thank you for any help here.

    1. Rick says:

      Hi John, Sounds like you have an interesting problem there. Could be a few things; I have seen this before and most of the time the culprit is the main controller board in the furnace. As all the thermostat does is basically close a set of contacts in a switch and that sends the 24V to the main board, which in turns starts the process going. If the main board starts having issues with the relays and switches built in, it can have this intermittent type problems.

      With that said, the Honeywell S8610U has a 5-6 minute timeout when there is a failure of ignition or is tripped off due to a safety, so you need to make sure you have completely come up to temperature and it’s actually shutting off from the stat. If it’s tripping off near the end of the cycle then it might appear to be restarting for no reason.

      To test this I would turn your thermostat way up so it will run for a long time without satisfying the thermostat and shutting off. If it runs and runs and never shuts off after say 20-30 minutes then it’s probably not from a safety or ignition problem. If it does trip off and the thermostat is still calling then you have others issues, from the flame roll out safety, high limit safeties, etc…could be several things possibly even the Honeywell ignition module too.

      If it didn’t shut off to a safety, then it could be either the main board giving the module the signal or the S8610U itself is faulty. I’ve replaced many of these…they can have short lifetimes of 5-7 years and it’s rare when I saw one over 15 years old. It’s quite a common truck stock item, but you might want to get a mechanic out to give it a thorough look if you can’t pin it down. BTW, here is the S8610U manual…you can see some of the troubleshooting guides…that might help you out.

      Good luck, hope that helps

      1. Thank you so much for the detail feedback. –rare that I have ever received such value. (I am too in a customer service role).
        Per your advise, I cranked the thermostat and watched everything run properly until the temp was reached.
        After reading your reply I realized that I didn’t know about other componets–I assumed that all the timing, etc was handled by the module. I popped the cover off the Honeywell fan motor limit thinginy. I can see it turn and go through the motions with delayed fan on/off. I see it continue to turn and eventually kick on the blower again at the end of its cycle. It doesn’t appear that it revolves smooth. –kind of jerky/springy. I adjusted the far left limit counter clockwise to see if that might stop its turn without turning the blower on, but I think I just reduced the time of when it turns on.
        I don’t know why this causes the blower to come on for a few seconds but I learned that, at least ther is no gas/heat loss due to this quirk.
        Do you think that I should replace this?

        1. Rick says:

          HI John,

          Yea sounds like it could be your limit/fan controller, they frequently go out due to age and the mechanical nature of those. All the newer type units use digital controllers, if you can find one for a good price might be worth a try, if not might be time to replace the unit.

  7. Derek says:

    Hello Rick,
    Very useful site here, thanks.
    My gas furnace cycles as it should when calling for heat. However, when the gas fires the rightmost burner flame flickers as if not enough gas or gas pressure it present. After a few minutes the furnace shuts down without reaching the heat setting on the thermostat. This process keeps repeating until the heat setting is reached, then it maintains the set point usually without issue since it is not moving the temperature more than 1 degree. What can I check to get the burner is firing properly?

    1. Rick says:

      Hi Derek,

      The best thing to do is to check your gas pressure coming out of the gas valve, you’ll need a special gas pressure gauge or manometer to do this, if your running NAT GAS, it should be at around 3.5″ WC (water column) The other possibility is a cracked or rusted heat exchanger, this pulls in air from around the heat exchanger and is higher pressure than the gas pressure so it will have trouble burning or staying lit. The other hopefully simple solution would be a plugged orifice in the gas train. Check to make sure all the orifices are cleaned, use a paperclip and poke it into each one if you can get to it.

  8. Jeramy says:

    I have a 1983 Bryant furnace the blower motor don’t come on until the flame goes out i checked what i could not known what I’m doing I’m hearing circuit board, flame sensor, thermal couple, all kinds of other things people are saying could be wrong looks like you know what your talking about so I ask what could be causing the blower motor not to come on with the flame

    1. Rick says:

      So does the motor work at all? can you turn it on manually with the thermostat? Could be fan/limit control with that old of a unit, if you can’t replace it yourself probably be a $500 service call, might be better to just replace the furnace. If you can manually turn the motor on and the unit will work I would do that and get some bids for replacement, if your not in a hurry you should be able to find someone to replace that for under $2500.

  9. Eric says:

    I have two problems with my 18 year old furnace. My inducer draft fan runs continuously (99% of the time). I suspect that the fan is being controlled off of heat exchanger temperature (not a time delay) and my heat exchanger temperature probe/limit switch is faulty. I have occasional flame roll out on my furnace, which I suspect could be due to backdraft in the vent line due to high winds, but wondering if there are other possible reasons for this to happen (faulty spark ignitor, etc.). Overall, the furnace runs well. I had it inspected and cleaned and the service guy said that the heat exchanger looked good. The flame roll out problem has me sleeping with one eye open and the inducer draft fan is loud. I’m planning to replace the temperature probe/limit switch in hopes that it’ll correct the inducer draft fan issue. I will add a cap to the exhaust stack that will hopefully lessen the impact of wind on the draft. We have five CO sensors installed throughout the house just in case. Any advice would be greatly appreciated!

    1. Rick says:

      Most inducer fans are controlled by the main control board with a built in relay, when a call for heat comes on the relay will energize, it’s common for these relays to get stuck, try banging lightly with the backside of a screwdriver on the relay and see if it shuts it off, over time the contacts inside get welded closed or even stuck open. The relays are generally part of the board and you usually need to replace the entire control board. After the call for heat has been satisfied the control board will shut of the gas valve and keep the inducer fan on for X amount of minutes generally, I’ve seen a few with temp controllers too but most of the time it’s just controlled by a timer.

      As far as flame roll-out, this can be caused by the wind, so try the vent cap first. It’s also common for the heat exchanger being cracked allowing in air from the blower fan and thus blows the flame out, but if you had it checked then it’s probably ok.

  10. Tony says:

    Hi Rick,

    I have a York Diamond 80 furnace with the Hunter model 44665 themostat, my problem is my flame goes out after about 5-10 minutes. I tried pressing and resetting the 3 rollout switches, sand and clean out the flame sensor…turn my furnace back on and it will heat up the house for another 5-10 minutes and the flame goes out again. Does this sound like my high limit thermostat is bad?

    1. Rick says:

      Could be a number of things from the flame sensor going out, they sometimes will still work for awhile, also flame roll-out, cracked heat exchanger, high limit…could be many things without directly looking at it it’s hard to say when this sort of thing happens, best to have someone look at it.

    2. Tony says:

      Did you get your York Diamond 80 fixed? I have the same model and I have a similar problem. The heater will come on, run for about 7 to 8 minutes then the flame will go out. Flame ignites again, it runs for a couple of minutes then goes out. This will happen 5 times then the heater will shut off and go into “lock out” mode with a code 8. However, the weird thing is that if the house is already very close to the thermostat set-point, and if the outside temp is only maybe 5 or so degrees from the set-point, the heater will work perfectly and hold the house at the set-point — it would never go into lock-out mode.
      I’ve replace the flame sensor but that didn’t help.

  11. Jeff says:

    I recently had problems with my combustion blower motor. Upon inspection, it had been ridden hard and put up wet…… ha the motor itself was almost seized up, and the switching unit from the centrifugal switch? had broken. I cleaned and lubricated the motor, and super glued the switch back together. This worked for a while, but the motor finally locked up again. So, I found an old furnace with a similar motor, and figured out the wiring, and replaced the motor. I had to use the old squirrel cage because of the fin direction. Well, now my furnace doesn’t seem to get as hot anymore? I’ve attempted to adjust the openings on the input to see if that is the culprit, I believe it is, and helped a bit, but still not as warm as it was? Please help?

    God Bless,
    Jeff P

    1. Rick says:

      Hi Jeff,

      I’ve seen where the wrong motor can pull more air or less through the heat exchanger, causing all sorts of problems. If you replaced a 2500/2800 RPM motor with a 3000 RPM motor then it will draw the heat out faster and you won’t get as much efficiency and it will appears to be blowing cooler air. I’ve seen this happen even with so called OEM motors that were mislabeled for motor RPM, was on a large commercial unit and all of sudden when it dropped to near freezing the unit couldn’t keep up, took many trips to figure that one out. There are some tricks you could do, such as slowing down your indoor fan, but if you can just try and find the right motor maybe?

      1. Jeff says:

        Thank you for your quick reply! I believe I have it figured out? There was a “sliding” piece of metal, designed to increase/ decrease air flow? Slide it one way and the opening got larger, the other smaller, more air vs less air, I ASSUME? As I have made that larger, it has increased in heat. I am assuming this is the right thing to do? I will check on the RPM’s tomorrow, I’m guessing they were the same? When I compared them, I believe everything LISTED on each was the same? Now, I’m just concerned if it’s too much flame, and/ or it sounds like it is a little out of balance with the new squirrel cage on it?! And that’s kind of strange, as if it was out of balance, it would vibrate the whole time it would be on, it presently just seems to vibrate for the 1st couple of minutes?
        I’ll let you know about the RPM’s, and thanks for the help! I’m a single disabled father, and every penny saved is every penny saved! lol

        God Bless!

        Jeff P

  12. Bryan Monaco says:

    I have a York Diamond 80 from around 1999. I am not sure of the model, but it is a small horizontal model. (my house is only 700 sq ft). The furnace works fine most of the time. The inducer motor turns on, the flames start, and then the blower motor. However on rare occasions the blower motor starts first, then the inducer, then the flame. It works fine, heats up and shuts off when it is supposed to. It may have been doing this for a long time, but I never noticed it until recently. Many times it will do this when I push the thermostat to command heat, shortly after the furnace has been running, but most of the it doesn’t do it. On very rare occasions it will have this problem randomly. I had the man out who installed the furnace and he has 35 year of experience and he can’t figure to out. He says the furnace is in great shape and appears to function correctly except of this occasional event. It happened while he was working on it Saturday and the computer didn’t flash any codes. The furnace worked properly the rest of the day and continues to do so. I didn’t notice this problem until my after man came for routine maintenance, last week. He checked the flame and the only thing that he messed with was he cleaned the flame sensing rod and blow threw a vacuum line. Any ideas.

    1. Rick says:

      Hi Bryan,

      The only thing I could really say is that it could just be an issue with the control board, sometimes they just get screwy and have intermittent problems like that. I installed a Trane furnace for a friends house. After 6 months it would periodically just start the indoor fan for no reason it would run from a few seconds to hours and sometimes would need to have the power cycled to reset it. I checked everything and talked with the Factory about possible solutions, after many attempts we eventually just replaced the control board under warranty and that was 3 years ago and he hasn’t had any problems since. Sometimes it’s the software code or who knows? That’s the only thing I could think of, if your tech checked everything else.

      1. Bryan Monaco says:

        I found the instruction manual for my furnace and was reading the theory of operation. I found if one of the limit switches was open the blower and inducer would run. I am going to try that fix tomorrow. According to the manual it should flash a “code 4” if that happen, but I am not flashing any codes. Maybe it only flashes when it happens. Since it doesn’t have this problem on a regular basis maybe I am not seeing the flashes.

        1. Rick says:

          If you have a door switch that cuts off the power, that will reset your control board. Depending on how old it is, after some time relays will start to get stuck on, if this happens and you can open it up while still running and you tap on it and it shuts off then you’ll know for sure. The same thing can happen with limit switches but less common, usually they just fail.

          1. Bryan Monaco says:

            My furnace man and temporally bypassed the auxiliary limit switch, inside the blower motor housing, and the problem went away. Apparently there is an intermittent problem with that switch because it might work normally 100 time and then screw up once, then work for days without a problem then screw up twice in a row. A new auxiliary limit switch is the solution for this problem.

  13. Peter Bakshi says:

    Hi There!!! I have a Carrier Gas furnace 58CVA/CXA . For the last few days, it is not working properly. The ignitor does not ignite of its own, however, manually, it does and furnace works well. Called a furnace guy and he advised me to change the ignitor and I did so, however, the results are same. An electrician friend of mine checked all the switches and other parts of furnace and feels that the CPU or the control board needs replacement. Could you kindly give your expert advise ?

    1. Rick says:

      Hi Peter,

      Did you replace the ignitor with a factory replacement? is this a HSI-hot surface ignitor?…get’s bright red etc…or a spark ignitor? if it’s the older style HSI and you touched it with bare hands the oil can cause it to crack the first time, also they are very fragile the ones that are white looking…hairline cracks are impossible to see and they won’t work, I’ve replaced many that looked perfectly fine.

      With that said it could be the board itself, it does happen… not much advice I can give on checking it out without getting my hands on it. They either work, they don’t or only sometimes:) The newer silicone carbide HSI, universal type ones are really nice and are not fragile, it’s still recommended not to touch them but I haven’t run into any problems yet with them.

  14. jeremy says:

    my furnace went out due to bad cage motor i replaced motor now furnace will start pilot lights and flame comes on but the motor is not getting a signal to come on what could be the cause? thx

    1. Rick says:

      Could be the fan relay, or fan/temp control if you have that, could be several items along these lines, hard to know for sure.

  15. Will says:

    Got an SJC Furnace from 1980, electronic pilot. Is there a way that I can adjust the time the blower motor runs after the stat stops calling? Currently its set to continue circulating air for 1.5mins after flameout.

    1. Rick says:

      Most timers are integrated into the control board, so you can’t adjust the time, but there are some thermostats that allow you to extended the run time. I have a Honeywell Vision pro thermostat but there are several companies that make ones that will do this for you.

      1. WILL says:

        No I am not describing heat-run-time. I am describing what every furnace ever produced does after heat is called off. Cool down. The fan circuit turns on after the stat reaches temp, and on my furnace built in 1980, before circuit board furnaces, it somehow lets the fan blow air across the heat exchanger for 1.5mins. I am aware that heat cycle run-time can be lengthened or shortened from the t-stat, of course. But this is not a part of that at all. I want to lengthen cool-down.

        Thanks for your response thus far…

        1. Rick says:

          Hi Will, most likely your fan stays on due to a thermodisc aka temp fan controller, this is a temperature activated switch it just probably takes 1.5 minutes to cool down below this shuts off. Most of these are non adjustable from 95-125 deg F placed near the heat exchanger, you can replace with a lower temp to make it run longer, or see if you have an adjustable one and set it to the lowest setting.

  16. Gigi says:

    Rick, this is the first site that I have been to that actually gives GOOD, responses to questions asked! Your Phenomenal! I am truly hoping you can help with my issue….my husband and I moved into our new home last Sunday….furnace was certified prior to buying…..it was working fine the whole time we were moving in. Sunday night (exhausted from moving)….the temp in house was 55 degrees and going down. I have a York Diamond 80, not sure of age. It has an auto pilot. The furnace will try to come on….I see spark, “randomly” smell gas, but still won’t light. The igniter will “sometimes” turn poker red. The igniter will attempt to spark/start 3 times and then the fan will kick on and blow (cold air) because the burners did not light. I attempted to get into or look in the lower cover but was not able to get it off to see if it was “blinking” codes. Now, I came home on lunch Monday….furnace working fine (no intervention). Worked all night fine. Then Tuesday night, get to the new house….52 degrees in the house. We replaced the thermostat thinking maybe it was not good (and honestly, we weren’t entirely sure how that one worked anyway), bought a Honeywell 5-2 programable. Installed, easy to use….still…no heat! Back to the furnace, nothing. Around 3:30am, I woke my husband to do something/anything to get that thing going…..he was able to “manually” light the burners (which I am told you are not suppose to do with auto pilots), but we were freezing. Came home on lunch yesterday….working fine….got there last night ….48 degrees in house! Tried for the longest time to manually light the burners again….no success. Woke up this am 42 degrees in house. Now I live in Michigan and the high’s have been below zero currently…heat would be a great thing! :). “Supposedly” the furnace guy is coming today to fix it….but he was also “suppose” to come last monday and was a no show. So in case Mr Furnace Guy doesn’t show….I really want to get this thing fixed today….I am going on very little sleep since we moved and I need sleep! lol…..any help you can give would be appreciated. Intermittently works…..then sparks but won’t light burners, intermittently igniter turns poker red, but not always, Cycles through to ignite, light, fan ….. fails on the light and then fan blows cold air. I hope this is enough info, not too “furnace” savy.

    1. Rick says:

      Hi Gigi,

      That’s allot of info and intermittent problems that your having. I’m a little confused by you saying it has both a spark ignitor and one that glows red this is called a HSI-hot surface ignitor, but either way it doesn’t matter a while lot. First let’s look at the main problem: it’s sometimes lights and works fine other times it will time-out after 3 tries which is typical, thus requiring you to shut the power off to reset. When this happens I usually check the gas flow on the outgoing side of the gas valve, not really easy for someone like yourself to do that, other than getting a long match or lighter and manually lighting the gas to see if you have enough.

      It sounds to me the gas valve could be the problem if it’s not lighting at all and the ignitor is working properly, this means if it’s a spark that the spark is big/hot enough in the right spot to ignite the gas. If it’s a HSI it’s will glow white hot.

      If the gas does light but then shuts down after a few seconds, then it’s not sensing the gas is lit, this is a common problem. You could have a multitude of different types of flame sensors, from thermocouples to thermopiles to HSI feedback….there could also be a flame roll-out sensor getting tripped from a real fault or just going bad.

      I hope your furnace guy made it there and is a competent mechanic to check everything properly, there are also other various issues such as the combustion blower, draft/venting problems, really could be many things that would be hard to help you with online.

      Best of luck… Rick

  17. Luxrider says:

    My furnace works at times and not so much other times. I have a open roll out light flashing. I turn off the power for 5 mins. some times it will run untill the the thermo setting is reached. When Thermo stat calls for more heat sometimes it runs as normal and other times The gas blower will start and the igniter will glow. Then as its about to restart the roll out code starts to flash and igniter goes off. The blower turns on and runs till you power down. All the roll out swiches seem fine. Is it possible the Circuit board is bad.

    1. Rick says:

      Are you sure your not having a flame roll out? also when heat exchangers go bad this will cause intermittent roll outs.

  18. Roy says:

    Hi Rick. I’ve got an interesting one that has stumped several technicians so far. My 80% gas furnace induced motor will start when the thermostat calls for heat. as soon as the spark igniter starts, the induced quits. When the sparkler stops the induced will start up again. This start/stop can happen several times until the pilot lights. Once lit, the furnace cycles normally. So far I’ve replaced or had replaced the blower control board, pilot ignition module, gas valve and 24v control transformer. Yet the problem remains. And I have diagnosed that the thermostat is not the problem by removing the stat and whimpering the red and white wires. No difference. The induced still stumbles when the sparkler sparks. Any ideas? Thanks.

    1. Rick says:

      Hi Roy, sorry it took me a few days to get back to you. I’ve had this happen to me before on a boiler, I had practically replaced everything trying to fix it. Eventually I found that it had several issues with the ground wiring in the main panel feeding the boiler. Maybe look into the power and redo the connectors and make sure the connections in your circuit panel are good and tight, other than that it has to be the ignitor pulling power away from the inducer if it has a capacitor that could be bad as well. Sorry I couldn’t help you any more, those are a real pain when problems like that arise….If all else fails you could wire a separate relay to control the inducer or replace the spark ignitor and put in a hot surface ignitor system, either way will cost more money unfortunately.

      1. Roy says:

        Thanks Rick. Now I have additional areas to look into.

  19. Mark Hike says:

    Hi Rick:
    My 98 gas furnace started to act up last week. When I turned it on, the draft inducer started immediately and a few second later the ignitor started the pilot, after a few more seconds the burner started. But the burner only ran for one minute or two, then turned off. The ignitor kept trying, starting the pilot, and continued to do that for dozens of times before giving up. (Sometimes, the burner was started and ran a few seconds then stopped). Then only the draft inducer is running.

    If I turned off and turned on again, the ignitor would started the pilot and did that a few times but the burner never turned on again.

    But if I waited for a few hours and tried again, I can repeat everything above, i.e. the burner can be started and ran a few minutes and then stopped, the ignitor/pilot kept trying a dozen times ……

    I have replaced the ignitor/thermocouple combo with a new unit and everything is the same. The temp limit switch and pressure switch all checked out OK.

    What else can be wrong?

    Thanks in advance,

    – Mark

    1. Rick says:

      Hi Mark,

      you could still be having a problem with your limit switches, I had mine opening about 15 deg F sooner than what it should have been, this caused short cycling starting and stopping but sometimes would work ok, it doesn’t happen often usually they just stay open but the bi-metal or refrigerant in them can go faulty or leak causing intermittent problems, try jumping the safety on temporary and see if it will cycle normal a few times and that will tell you for sure. Another problem could be in the ignition control module or if integrated into the main control board could be something there.

  20. Rebecca says:

    When my ’97 York furnace is not on for a period of time, the first time it kicks on a significant puff of natural gas smell comes out. When the furnace is running frequently (like in the winter) this doesn’t happen. Seems like a glow ignitor, not a spark ignitor. Furnace has been checked for leaks and no leaks. Heat exchanger is also fine, I am told. Could something be sticking or plugged?

    1. Rick says:

      Hi Rebecca sorry for the delay, I wrote this already but it didn’t get posted the first time. Sometimes when a furnace starts for the first time after sitting awhile the glow ignitor may take awhile to heat up before it ignites the gas, or it has to try several times usuallu burning off the dust on the ignitor or flame sensor, I’ve had this even with mine before and you can smell the gas…it is sort of normal but is one of those things that’s hard to pin point the root cause. If there is a draft it can pull the gas out of the burner area as well.

  21. Robert Mittrndorf says:

    My furnace won’t start in the a.m. HSI glows, I can hear the gas valve open, but no flame and igniter goes out. This happens three times and the fan starts runnning. If I shut the furnace off and start again, the flame ignites, usually 1st or 2nd try. Flame sensor is 3 months old, I have removed and cleaned it and still have the problem.

    1. Rick says:

      Hi Robert, sounds like a gas valve sticking, resetting the power is usually enough to open or close any coils on the valve and cause it to reset a bit…next time it does this, after the first non ignition try tapping on the gas valve with a screwdriver or wrench and see if it lights up without having to power down, if so then you’ll need to replace the gas valve.

  22. Sarah says:

    My gas furnace kicks on when it is really hot in the house. I notice it does this when it is over or around 90 degrees outside. I live in a single wide mobile home. When it did that today I just turned the switch to off. Why does it kick on like that?

    1. Rick says:

      Hi Sarah, do you have an older mechanical (non-digital) thermostat? If so that is most likely the problem, I can’t remember which brand but they are older and if it got too hot they would just keep winding to the point of turning back on. If it shuts off by turning the thermostat off, it’s very likely the thermostat anyway and it should be replaced.

  23. Kent says:

    Hi, I have a 16 y/o York Diamond 80 down draft gas furnace. Recently and intermittently the auxiliary limit switch (160 degrees) on the blower fan is tripping. It usually happens when starting from cold after being switched off for a day or so but once or twice it has happened during normal operation’s on/off cycling. I changed the filter even though it was new but no change. The switch is a manual reset type and I had reset it many times. I suspected it was faulty due to age but have replaced it and the problem continues. I’ve monitored it closely for a number of cycles with the burner door open and the combustion blower, HSI igniter, gas valve all come on/off and work properly. No sign of rollouts. The one time when I caught it right after the auxiliary limit switch tripped open (failure mode LED 4 flashes, only mode ever displayed) the blower fan chamber cover was indeed warmer than normal so I suspect the blower fan motor is not switching on at times. But after resetting the switch and waiting a few minutes the furnace returned to normal operation and has been fine for several days now. I’ve inspected the wiring at the control board, the motor, etc and all looks to be secure & in good shape. I am speculating either the control board, motor or its starting cap has an intermittent issue, but while I have some electrical diagnostic “game” I am by no way a furnace expert. Is there a way I can confirm what specific component is at fault? Other than catching it at the moment of failure with a meter on the motor feed (fat chance) I’m stuck. Any advice? Thanks for any advise you may have. BTW – this is a great site! Thank you, Kent

    1. Rick says:

      Hi Kent,

      Since you have replaced the limit switch, I would have to agree it’s most likely a problem with the fan, whether that’s from the control board, relay or the motor itself is the question. One way to test if it’s just the motor over heating etc would be to crank up the heat so it’s on for a long time over 15mins continuously at least, even up to an hour if need be. When I do this sort of test I will amp out the motor leads at startup and then every 5 minutes to see if they are climbing, also I put a temp lead or digital thermometer right next to the limit switch, this usually requires drilling a 1/8″ hole. That way you can check to see if the temp is climbing and it’s working as it should or tripping too early…just a few ideas if you have any of the tools. Also make sure all your ductwork is clear and opened and same with your return air (filter) sounds like you got that covered.

      When motors start to go out they can be incremental likely due to bearing wear and/or start/run capacitors failing, plus a few other things…if it sounds like it’s starting slower…then that could be the problem, the motor will start and run for X minutes before it overheats and trips off or a bearing seizes enough to shut it down and then when it cools down it can restart just in time for you to see it working properly 🙂

  24. harrison voigt says:

    Startup problem with my 25-yr-old Kenmore gas furnace. Replaced two parts, both OEM: the inducer motor (did not fix problem) and the pressure switch (ditto). The flue pipe is checked, clear, and well-sealed. The hoses for the switch are clear and tight. Thermostatic call for heat works fine, and the inducer motor starts and runs — but the ignitor (also newly replaced) does not fire even after five minutes. Troubleshooting suggestions are most welcome!

    1. Rick says:

      Hi Harrison,

      That’s an old unit you got there, if you get 15-20 years out of a furnace that’s a good one. Are you getting any gas coming though the gas valve? can you smell anything? I would test the power leads going to the ignitor to make sure it’s getting power…most likely the ignitor control/board based on how old it is. Some control boards are all-in-one older units may have separate ignition controllers…any of the old Johnson Controls can fail at any moment or last 30 years.

      1. harrison voigt says:

        Thanks very much for your reply, Rick. Actually, the furnace is 36 years old!

        Turns out that the problem I had was the furnace repairman who first came to troubleshoot the unit. He told me that the inducer motor was bad (correct), and that maybe also the pressure switch. To check the switch, he pulled the hoses and checked the pressure with a manometer which showed intermittent failure. Gave me an estimate of $750 to replace both the inducer and the switch — but also said that there might still be a problem. So I passed, replaced both parts myself along with the igniter for a total cost of less than $300.. Still no go, inducer ran and ran but no ignition. I did some research, and (for me) brazenly tested the pressure switch by sucking on one hose, and found that this could force ignition. So I tried one more thing, reversing the switch hoses. BINGO! Everything now works perfectly. Yes, it was the repairman who reversed the hoses when he reattached them.

        Another story from Furnaceland.

  25. Keith says:

    Awesome posts!
    I have a timer/fan problem. After reading all your great suggestions and problems for everyone else, I decided to drop you a note.
    My furnace starts and runs but after a bit it shuts off. I cleaned the flame sensor and when it ran again, I noticed the temperature wheel spins right up to 150 degrees and shuts off.
    Any ideas why my fan is not starting up to keep the temperature regulated and prevent shut-off?

    1. Rick says:

      It sounds like the fan is not starting because of the time/temp unit controlling it, depending on how old it is…the limit then shuts off the gas. The old ones used a fan/limit control box and the newer ones use a bi-metal disc, either way as long as your motor will work separately it’s whatever is controlling that…sorry to be so vague.

      1. Keith Sanders says:

        The guy that installed a new fan this summer used a multiple speed fan and he said he needs to come over change the two speeds he picked and rewire it. Does that make sense?

        1. Rick says:

          Hi Keith,

          Usually when you install a multi-speed motor there is 3-4 speeds i.e. Low Medium Med-High High, the speed taps are then connected to the fan control for each the Heating and Cooling cycle High for cooling and low or med usually for heating. He could have wired them to the wrong ones and just wants to fix his error? is he charging you for this? He shouldn’t if it was a mistake, once set the speeds should never have to be changed unless you’ve done something to the system to warrant more speed, like adding another register for an addition, and it’s the farthest run for heating you may need a higher fan speed to get the air there.

  26. Sam Macrina says:

    Hi I am replacing a flue blower on a heil furnace. I took the old one out several months ago and I did not mark the wires to the motor or to the air pressure switch. Can you help? I do have pictures I can send.

    1. Rick says:

      Hi Sam, I would be happy to help. If you want to send me the pics and if possible a picture of the wiring diagram I’ll do me best to explain. Just send email to Rick @ ricksdiy DOTcom

  27. pete black says:

    I have a Amana Furnace, changed the control Board, When call for heat, Hot surface ignitor comes on but gas valve does not open every time. It is getting 24 volts every time and the gas valve clicks every time but does not open. When it does open the furnace cycles but will not come back on. The control module is a 50A55-843 White-Rogers Universal and the red diagnostic light is not on. it does not come on at all. do you think I have a bad control module?Help

    1. Rick says:

      Hi Pete, sounds like you might have a few issues. I would check your grounds really good and maybe even add an additional one if you can for the gas valve and the control module. It could be bad, why did you change it…same reason?

  28. John says:

    I have a Kenmore furnace and it will not start it flashes a code after a little bit of trying to start. I thought it was the control board. But I wanted to make sure so I hooked a cord up to the inducer motor it has to prongs on it so I hooked it up to those terminals to no avail also theres a green wire running around the motor from one end to the other guessing thats some kind of ground. Anyway can I assume its the inducer motor?

    1. Rick says:

      I would check and see what the code tells you, usually there is a code list on the sticker inside the unit.

  29. Don L says:

    Great resource page, Rick. I have a question about a Bryant Plus 80 that seems to have a cycle issue in that after the inducer motor, ignitor, and gas solenoid sequence, the has shuts off before the main blower kicks on. The main blower then runs for the “blower on after power up” portion, and the code light blinks the 12 code. It seems like a sensor or timer issue, but I’m not sure where to start. Any advice would be appreciated. Thanks.

    1. Rick says:

      Hi Don, Check and see what your 12 code says, should be on the sticker inside the unit/door could be a flame sensor/flame rollout issue causing it to shut off the gas. This is most likely the cause, check that and then get back to me. Thanks

  30. Martin says:

    I have an older goodman PGO outside combination unit. Heat worked fine two weeks ago, went to turn it on Friday and nothing. It is getting power to control board and the inducer motor is running. Getting power to the intermittent pilot control box with none coming out, both safety sensors have continuity (rollout and temp sensors). Would I be correct in suspecting the pilot control box?

    1. Rick says:

      Hi Martin, I would shut the power off to the unit and then try restarting it and see what happens, most Ignition modules will try to start 3-4 times then shut down and require a power down to restart, check and see what happens then…if nothing then it likely is the ignition module

      1. Martin says:

        Rick, I replaced the ignition control box. Now I can actually hear the box intermittently clicking, I’m assuming for the direct spark ignition. I see no spark through the inspection window. Took old ignition box apart and cleaned it and reinstalled. Same deal, inducer motor turns on, ignition box clicks for several cycles then nothing. Kinda stumped here. Any ideas? Thanks for your time.

        1. Rick says:

          Hi Martin, The control box can still be bad if it’s still clicking, you can check the high voltage side BUT BE CAREFUL YOU COULD GET SHOCKED… by pulling off the end of the wire from the sparker and holding it close to ground/metal surface using a pair of insulated pliers and gloves, if it’s sparking there then the sparker has an issue, some are just a metal rod and others are spark plug types. If you get no spark you can check and see if the wire is bad, pull the high voltage spark wire off from the ingnitor box, use a length of 12-14 gauge wire and ground one end under a screw, then use the pliers and heavy gloves to hold the wire near the spike/terminal on the ignition box and cycle it on, if you see a spark jump then you know your ignition box is good, probably have a break in the ignitor wiring internally which is common.

          1. Martin says:

            I went out to test it and the ignition box wasn’t audibly clicking once I powered it back up like it was last time.. Swapped in new one and no change, still can’t hear it. Could this be an indication of something else bad in the sequence before the box?
            The ignitor/sparker wire looks like a solid rod with an insulator. The wire runs straight into it so I can’t detach it. I suppose I could cut the wire to test it and simply replace it. Throughout the process of messing with it, I’ve also yet to hear the gas valve open or purge.

          2. Rick says:

            Hi Martin, The gas valve should turn on right when the spark ignitor starts, if you don’t smell any gas or hear the valve open then your probably not getting that far in the sequence, on a call for heat the first thing it will check is the safeties as it turns on the inducer motor, most furnaces use the air switch to verify the inducer is running, if there is a problem with the air switch the inducer motor may continue to run but it will stop the starting sequence, try rapping on the air switch a bit see if anything kicks on, check the air tube etc…other than that if all the other safeties look good may need to have a tech check it out, could be the main board…

  31. Tony says:

    RICK I have a Bryant 395B gas furnace with a pilot burner assembly. when I start the furnace ,the inducer fan come on the pilot lights then the burners come on. After the blower comes on , in about 8 to 10 sec the main burners go out, then the spark generator relights the burners, this happens 4 to 5 times then the burners stay lit until the end of the cycle. some time it does not do it. could it be the BI metal switch in the pilot assembly or something else???

    1. Rick says:

      Sounds like your getting an intermittent fault, could be a grounding issue with the ignitor/flame sensor or a safety that is starting to fail. The best way to isolate is to use a jumper wire to “TEMPORARILY ONLY” jump the bi-metal safety and try it several times but never leave it like that and see if it fails, if it fails then it’s not the bi-metal safety. Most the time I just replace the ignitor and safeties at the same time since the safeties are usually not very expensive but the time to disassemble everything might as well replace them too.

  32. chris riley says:

    Hello I have an old Bryant furnace (1980s ) and the problem i am having is that the furnace will run through the heating cycle perfect until the cool down cycle begins. fan will run for a few minutes and will shut off and directly after fan shut down fan will cycle again for a second or 2 and may re pete 3 times and then shut down. I am thinking a possible fan limit switch hanging up. any help would be great and thank you

    1. Rick says:

      Hi Chris, That definitely sounds like the fan limit control alright, I’ve also seen issue with control boards doing this too on several units lately. Try tapping on the fan control right after it shuts down and see if it comes back on.

  33. Paul says:

    I have a similar sounding problem with a Bryant plus 90 gas. The inducer motor turns on, the ignition turns on, the main flame turns on. The normal operation is that soon after the main flame is on the blower motor starts. The problem is that the blower motor does not start. After several minutes, the main flames turns off, and then the blower motor starts (a cool down sequence?). What should I look for to get the furnace back to normal operating? That is the blower motor running wit the main flame.

    1. Rick says:

      Hi Paul, I’ve had this before in several different ways depending on how it’s wired, on one I had a 4 speed motor and the med-hi speed was used for normal heating operation but med-low speed for the cool down? the med-high speed had shorted out but yet the motor would work on other speeds(very unlikely btw) The most common problem is the flame sensor signal that controls the fan motor relay, either the flame sensor has an issue or the wiring from it to the control board, look at the connections and see if you can find a loose stake-on connector etc. Also check the grounds, as this is used in the flame sensor circuitry, sometimes just backing the screw off the ground and then re-tightening is enough. The cool down sequence is initiated off a separate control, either temp or simply the heating cycle shutting off. I would try turning the fan on all the time if it is wired that way through the thermostat and see what happens also, to be sure it’s not a fan speed issue. Good luck!

      1. Paul says:

        Thanks Rick, I will try the tips.
        I talked with a furnace guy yesterday. My Bryant was installed in 1987 (so its as old as my youngest daughter).
        the furnace guy says those furnace have lots of issues with the water from the condenser. The fittings corrode, leaks etc. Mine has those issues and I have had to apply goo in several places. The water will cover the circuit board connector and cause problems. So I am drying the furnace out with a fan and will reapply some more goo. By the way, Shoe Goop, is some pretty good sealant. It is sold as shoe repair but it is also very good at many other seals and repairs. I can run the fan in manual and have heat. I hope a good drying out will help.

        My question is: Have I got about as much life as I could expect from this furnace? Or should I look at getting a new control board?


        1. Rick says:

          Paul, Yea your closing in on 30 years, 15-20 years is good, at about 30 you should be budgeting for replacement. If your already have those kinds of issues then I personally would replace it if I had the $$, Don’t worry about buying the high end ones, the lowest priced HE unit today is far superior to anything even 5-7 years ago. I would stay away from ECM motors, go for variable speed…still seeing lots of issues with the ECM motors.

          1. Paul says:

            Update to my furnace problems.
            The furnace guy I prefer told me he would not be able to replace the furnace until after the first of the year. He is that good. So I decided to attempt repairs.
            I sealed the water leaks with Shoe Goo. I ordered a replacement control board from HVAC outlet. The board arrived in two business days.
            I replaced the board this morning and the old Bryant is alive and working as intended.
            Total cost 5.00 for shoe goo, 88.00 for the control board. About an hours work. 30.00 for the furnace guy to come over to tell me he is too busy to work on it for three months.

            Thanks for your advice Rick!

  34. Jim Sherry says:

    I have an Armstrong Ultra SX 80 furnace. It cycles three times and then shuts off. It has a pilot and flame sensor assembly which I did just replace thinking that the flame sensor was bad and causing it to shut down. I still have the same issue. Could it be a bad Honeywell S8600C control module or a faulty gas valve? I’m at a loss and could use some help.

    1. Rick says:

      Yea it could be the Honeywell control, try all the wire connections and make sure they are tight maybe even add another ground…these are notorious for those issues…if that don’t help look at replacing the module…btw I’ve seen new JC and Honeywell control modules bad brand new in the box…

  35. Steve says:

    I had a control transformer go bad and melt some wires on my unit. I replaced all the burnt wires and the transformer now everything works (A/C and fan) except the burner will not light. The pilot stays lite but it just won’t light the burner. I was wanting to know if the general purpose relay can go out but yet still run the A/C?

    1. Rick says:

      Steve, Yeah it could, or something else actually caused the transformer to fail. If it melted some wires that means it probably failed on the high voltage side which usually doesn’t happen, most of the time the low voltage side fails. When I do see the high side fail it is usually from another part failing. I would look into the relay, many are built into the control boards though…tough spot good luck.

  36. John says:

    I have an Ultra SX80 furnance.
    The inducer blower starts, but the HSI doesn’t glow.
    I assume it’s getting stuck on step 3 of your sequence, where is it checking the safeties.
    I can hear the pressure switch for the inducer work, and I’ve double checked it by jumping it out of the circuit.

    What other safeties should I be checking?
    Or it is likely that the HSI has failed in some way?


    1. Rick says:

      Hi John, Sounds like the HSI unless you know for sure that it’s a safety, if those check out then try the HSI, look at the universal replacement Nitride Ignitor kits, they are around $25-45 and are really the best thing going these days, I don’t even try OEM unless they are a special type…but most of that is just the commercial units, most residential ones can use the Nitride ignitors. I’ve used the White Rodgers, Supco, 120V versions and they work great. Also known as Glo-Stix. They are not fragile like the older style HSI, where oil, moisture or vibration can cause them to crack.

      1. John says:

        Hi Rick,

        In the meantime, more has transpired.

        I checked the voltage from the “smart” valve, and found 24v to the HSI. No visible evidence of a problem with the HSI, and it measured 14 ohms, which was lower than I expected.

        Even though I couldn’t diagnose a specific problem with the HSI, I purchased a replacement and installed it. It worked for about day. Now it again doesn’t glow. Back to square one.

        One thing I did notice is that the HSI didn’t seem to stop glowing after accomplishing the ignition cycle. I would have expected that “smart” value to cut the power to it. I can’t confirm that, because it’s not glowing again.


        1. Rick says:

          John, Yea I’m not an expert on the smart valves, seems the smarter they make things the harder they are to figure out…:) If the new one had worked for a while then stopped maybe whatever is supposed to shut it off, smart valve etc is failing and by keeping it on it has failed again. At this point I would probably call somebody in rather than trying to replace it again because something else is going on there.

          1. John says:

            Hi Rick,

            The with new HSI installed, the furnace has a new behavior:

            1) Thermostat calls for heat
            2) the HSI lights the pilot and then goes out (repeat 5 times)
            3) finally, the main burners light, and then go out after 2 seconds
            4) the HSI lights the pilot and then goes out (repeat 10 times)
            5) the main burners light and stay lit

            I’ve checked the vacuum line to the pressure switch.
            Any ideas on how I can track down what is confusing it?

          2. Rick says:

            Hi John, That could take awhile. What a headache, could be an issue with a safety kicking on/off like a snap disc or a rollout switch if there is one and then it could also be the flame sensor…no real good way to do it other then selectively picking out one at a time and jumping it out and then seeing if there is a problem or not, if that doesn’t do it you move onto the next one. Sorry I couldn’t help you out more, problems like that are real frustrating even for seasoned journeyman.

          3. John says:

            The flame sensor is part of the same assembly with the HSI, yes? I just replaced that, so hopefully I can cross that one off the list. I’m not familiar with a snap disc or rollout switch. Can you give a short description of what they do, and where to look for them?
            thanks much!

          4. Rick says:

            The snap disc/flame rollout will usually have 2 push on connectors with multiple wires running through each one, here is a link to a picture of two in series; you cna also google to see what others look like but they are basically the same. some have manual push to reset types others are automatic, they open at say 180 deg F and then close at 140 deg F, but sometimes they can be erratic and open at a much lower temp, or you could be actually having a much larger problem like a hole in your burners.

  37. Drago says:

    Hi Rick.
    I have high efficiency furnace.Meridian:Model #ADA080NH3R
    Ser #040421004
    My problem is when thermostat calls for heat,blower motor turns on and runs for an hour or two before ignites the flames.We changed the spark igniter and gas valve but that didn’t fix the problem.Any idea what’s wrong.

    1. Rick says:

      Need more info, when it first calls for heat is it going through a start up sequence? sounds like it is but then cycling off after non starting, then after a timeout period it tries again and then it works.

  38. Raj says:

    I have Bryant gas furnace. When thermostat calls for heat, inducer motor starts. After few minutes igniter turns on and flame starts but it stops right away. This process continues for 4-6 times and after that furnaces starts.
    Sometimes It start normal and some time it does as I explained in the beginning. I also noticed that after heat call motor starts and stops. After few attempts flame will start.
    I cleaned flame sensor and replaced pressure switch.

    1. Rick says:

      Hi Raj, As with many of these it depends, sounds like the flame sensor, but could also be a loose connection or ground issue with the flame circuit or safety circuit, you really need to check everything when your having intermittent problems like that, sorry.

  39. Rick Z says:

    Hi Rick,
    Thank you for helping people out to deal with the trouble from the furnace.
    I got my gas furnace ignitor and the module replace last winter. The problem was discovered after opening up the module circus board was: one solder spot cracked. That’s why the ignitor sent sparks when I tap the module, and no sparks if I left it sit still. The new module is White Rodgers 50D50 – 843. Anyways, the new parts worked fine last winter.

    This year, the furnace with the new part worked several times when temp was bit low. We had a furnace cleaning last month and the furnace worked fine when the tech left. Then we reset- brought down the temp from thermostat and the furnace stopped. It seemed everything was fine at that moment. Then we went for a trip for several weeks. When we got back, felt a bit cold in the room and brought up the temp at the thermostat, then the furnace fan started running as usual, but, no heat. I went down and remove the panel. Heard the module tried to make the ignitor to spark. Usually, I can hear a banch of dadadadadada sounds, then it lights the pilot. But this time, I could only heard one Da, then it stopped. I watched the LED on the module, it stayed on, no flash. I waited for 2 minutes, LED still solid on, no flash. While I was waiting, I heard another Da, but, it didn’t light the pilot. The fan kept running itself, nothing else happened. I turn off the power switch, re-tried several times, disconnected and re-connected wires, removed the chip on the model and re plug the chip,checked ground wire and re tightened it, brushed the ignitor tip and sensor… still no spark. It sounds like the signal or voltage was block and couldn’t generate a spark. I could smell a bit gas for every try. What could the problem be? Many thanks. Rick Z

    1. Rick says:

      Hey Rick, Sounds like the ignition module again. This can happen with any brand. I had one customer that every winter for 4 years I had to replace the module at the beginning of the winter, then after the last one it has worked great for 7 years..go figure. I was able to claim warranty on most of them maybe you can get the same thing. If it’s powered up and no “DADADADADA” then something inside is faulty. G’luck hope the next one lasts for you.

      1. Rick says:

        Hi Rick,
        Just replaced the module. It works.The indicating LED is not always trustworthy. In my case, the light shows normal as the instruction described. But, something wrong inside the module. You are the master !!!

        The last module was replace by a company 2 years ago. They don’t honor the warranty any more because I didn’t buy the module directly from the supplier.

        This time I bought and replace the module by myself and I can hold the warranty if anything goes wrong.

        Thanks a lot.

        Rick Z.

  40. Kurt Mayer says:

    I have a Goodman GMP125-5 80% furnace with a SVB-80 direct vent blower. Problem is when the thermostat calls for heat the furnace intermittently will not respond ( the inducer motor does not turn on). When the inducer does turn on the furnace will go through sequence just fine. When it doesn’t respond to a call for heat, if you cycle power to the furnace ( emergent on/off switch) furnace then responds just fine. The SVB-80 has a inducer as well as the one In the furnace , what is the sequence and where do you think my problem exists.


    1. Rick says:

      Hi Kurt, Could be a number of things, but if your having to cycle the power sounds like something is locking out, this could happen after the last run of heating was on, or could be an issue with the electronics of the main board, Goodman did have a bad run of control boards on their units put out around 2005-2009…I had many with erratic control problems and failures in less than a years time…so check to see what year it was made, they did have a recall.warranty thing going for awhile.

      1. Kurt Mayer says:


        The furnace manufactured in 1994 and has had this issue for 2 years now. Is there a way to check the control board or is it a just change me and hope ? If I do change the control board is there any other set up to be performed ? If so I don’t want to get in over my head, I’ve fixed the hot water heater a couple of times, but that’s not a furnace.


        1. Rick says:

          Hey Kurt, It may prudent to have a tech come out and check everything, like I said you might have a few different problems. Like the thermostat, wiring etc. Sorry I couldn’t be more help.

  41. Clarke Blanton says:

    I have a 21 year old RUUD UGVH-07EAUER furnace. This is a horizontal unit in my attic. Every other cycle it skips the igniter phase and blows cold air for 10 to 15 minutes. The next cycle it will heat up again to the stat setting. For background, I can’t shut off the heater from the stat. Fan set on auto. I have removed the 4 year old Honeywell thermostat face and the unit continues to run. I have removed the red and white stat wires from the furnace control board and the unit continues to run. With the red and white stat wires removed from the control board the furnace will not start back up. I have replaced the fan limit and igniter element with no change in operation. I have checked the length of the stat cable wires and can see no problem that would create a shorts.

    For a good cycle I get two green lights, power and OK, steady on. The igniter element heats up, the gas fires and (the amber diode starts to blink throughout the cycle) the blower comes on. The next cycle skips ignition and goes straight to blowing cold air with two green lights on but no Amber diode (blinking or otherwise). This started about a month ago and I have tried it with and without the filter in place. The flame sensor appears to be clean.

    Before I replace the board I thought it might be helpful to check here.

    Thanks for any help.


    1. Rick says:

      Hi Clarke, First when your talking about it going straight into running the blower that usually comes from a temp-limit/fan control. If the switch is stuck or sticking then it’s trying to say I’m still hot run the blower for 15 minutes to cool me down. Depending on your sequence of operation if this is stuck on it may cause it to skip over the ignition sequence all together.

      But…. Your right to assume the probability of it being the main board, does this have a separate ignition control? if it does look into replacing or banging on that…sometimes it’s as simple as a faulty ground or loose push on connection. I don’t think I’ve mention this before but if I have an intermittent issue with a ignition module and there is enough extra wire I will cut the terminals off and redo them. I also will add in grounds as the ground is what is used to sense a flame most of the time. If the ignition control is integrated into the main board it may just be the relays getting old and sticking. 15 years is about the normal life span for the electronics.

      Let me know what you find, Happy holidays!

      1. Clarke Blanton says:

        Thank you Rick. I did replace the fan limit control sensor and I bought a spare too. All three measure between 2 and 3 ohms across their terminals and the replacement made no difference in operation. I went ahead and ordered a new board yesterday and will install it this weekend.

        I am not aware of a separate ignition control on this unit and will have to check the schematic.


  42. John A says:

    I have an older Moncrief industrial furnace that is giving me fits. It has a rodgers white auto pilot on ti and that is a new old stock unit, now. What it is doing is it lights the pilot for a few seconds then tries the burners. They light and then it immediately shuts them off and starts all over. It does this dozens of times and then stays lit. I just replaced the mercury flame sensor and it worked great 5 or 6 times and now is back to the cycling. This thing is pretty basic, so what is it mad about. It was not used much in the past, but the building is being used for a different business and need the furnace to not drive them crazy. This beast has 10 burners and dual fans. It was checked out a couple of months ago and the tech said the flame sensor may need cleaned. I did that and no luck so I replaced the mercury flame sensor assembly and still the same operation, Stone cold from being off it seems better. Any help would be appreciated. Thanks!

    1. Rick says:

      Hi John, look at the grounds for the flame sensor or simply try replacing the IGN module…sounds pretty simple in operation

      1. John A says:

        I will check all of the connections out. It worked for about 6 cycles, 7 hours or so great after I replaced the flame sensor. Then got its old attitude back again. Being 18 ft. in the air is also a challenge. It lived in a building that wasn’t opened much for about 5 years, and the humidity was fairly high, before that it didn’t get used 10 times in 18 years. I suspected the points in the flame sensor were corroded, but I don’t think that is the case. Is there a wiring diagram that can be found for this simple system? Thanks for the help!

        1. Rick says:

          If you can get a partial model number you might be able to google something, even look at images for scans of wiring diagrams. At my former employer I was well known as the trace/replace guy. I had the (un)?fortunate ability to be able to trace out almost any unit mostly older large commercial package units and recreate the wiring diagrams. I felt like it was a bit of mystery/treasure hunt to figure it out and then write it all down. We had an old Chrysler as in the car company unit that caught on fire. Everything was gone and most of the wire had to be replaced. The cost for a replacement was around $20K and a month out during the winter. I was able to rewire and make new diagrams in 3 days…but it is a hands on job..so sorry I can’t help you there.

          1. John A says:

            Hi Rick,
            I finally got to check things out. There is a White Rodgers mercury flame sensor, which appears to be working correctly, the valve, the coil box (which has only two connections), a power box with the transformer, and the box which controls the fans. This thing has dual fans and 10 burners.I cleaned all of the connections and reassembled. The only thing I didn’t mess with was the fan controller box. Here is what it does. Twice upon reassembly it worked flawlessly. Then upon the next call for heat, it fired the pilot perfectly, and when the flame sensor switched, the main burners lit and immediately all went out. It kept doing this, 10 or 12 times and then stayed lit, including the pilot light. I tried to fake out the flame sensor with a propane torch, as it seemed like the main burners blew out the pilot and everything. But that was proven false with the torch. The gas valve shut down as soon as the flame sensor let it call for the mains. I didn’t check voltages, but is it possible the contacts are dirty that shut the gas off in the temperature control box for the fans? It is the box that shuts things down upon overheat. Or possibly low voltage and the extra load of the main burner solenoid is too much? I have model numbers for the ignition box and flame sensor. Any help is appreciated!!

          2. Rick says:

            Hey John, it sure sounds like an issue with the flame sensor or controlling unit…is that what your referring to as the coil box? When your getting tripping like that I just replace the ignition module, flame sensors and ignitor and that usually takes care of it…for all the trouble it’s causing just try and get as much changed out at once if you can…but that flame sensor would be my first choice…I’ve had brand new ones not work…not a big fan of that type.

  43. Clarke Blanton says:

    The board seems to have done the trick and the unit is operating very well.


  44. Brian Bourassa says:

    Here’s a problem I have been having for years and it’s really getting on my nerves. I never know, when leaving my house unattended, that I will one day return to find a frosty house with water pipes burst…etc.

    My oil furnace was retrofitted with a natural gas burner nearly ten years ago. The oil burner never had the issue, but with two different natural gas burners I continue to have this problem. When the temperature gets to about -6 or below, the burner fails to ignite and it locks out. Oddly, I can then go and reset the burner and it starts up first time. 35 min or so later, after the water cools and the thermostat calls for more heat the same situation occurs again.

    Service men have come and gone many times over, replacing this and that, tweaking this and that, but never fixing the problem. The primary variable is cold, real cold. An other possible variable may be the size of the exhaust flume. When the natural gas people put in the burner they also put in a “liner” inside the slightly large flume. I don’t know why they did this and do not know if there could be an issue with this configuration.

    I am about ready to contact some upper-level people at the gas company and threaten to switch services, but want the best-of-the-best brains and service techs to have one last crack at this.

    Since I am selling my house soon, I really want this taken care of.



    1. Rick says:

      Hi Brian, I had a couple ideas…Luckily I never have to deal with temps around here like that but my wifes family is from Northern Minnesota and we frequently visit in the winter…-25 and Ice fishing is fun but no thanks to working in those temps. First you need to make sure the sequence of operation is normal and that everything is firing as needed, try and see at what point it cuts out and then let me know.

      One theory I suspect the cold temps are causing the flue gases to not vent properly and causing it sit low around the burners and not light all the time thus getting locked out..but you would think that resetting it wouldn’t help…so another option could be with the gas valve, do you smell the gas? or how about the air switch? it’s possible that freezing temps coming down the flue can freeze the fragile air switch diaphragm. Still you would think that “electrically resetting” this wouldn’t help much as these are physical issues…

      So I come down to something electrical that’s negatively affected by the cold, possibly in the control section. Hopefully your furnace is easily accessible, try this experiment; Get a drop light or clamp on style light fixture and place a 60-100 watt bulb in there. Place this in the control cabinet or if you can’t close the door pointing to the control section and or gas valve area depending on configuration. You just want to keep it above freezing without getting too hot in there. Do this when it’s really cold and you have been repeatably having to reset the unit…and see if it keeps working.

      If that works then you can at least try and identify which part is actually the culprit… by directing the heat at each component. I’ve had to do this a few times over the years on various equipment. I’ve had air switches freeze due to the hot gas condensing moisture in the flue and running down into the switch port, plus a few frozen gas valves from over demand on commercial equipment drawing in too much moisture due to improper gas piping.

      Good luck, sounds like a SOB:) all else fails just replace the entire unit or sell the home and plan on replacing it at inspection?

      1. Brian Bourassa says:

        I’m not a furnace repairman, but I have worked on multimillion dollar semiconductor equipment. That said, it still doesn’t make me an expert in anything!

        “First you need to make sure the sequence of operation is normal and that everything is firing as needed”
        I’m sure this has been done by the many repairmen that have frequented my home since its installation, and twice this past week.
        The Thermostat, calls for heat, the furnace chamber is purged of leftover gas (or so it sounds like it is doing this). Has it been verified…?? This is what is supposed to do, and you would think that this would take care of the cold dense air from the flume. It fails, essentially during the actual attempt to ignite.
        The gas valve was replaces two episodes ago, the regulator was replaced last night. There was a small section of piping (about 4in) between the gas valve and the burner that was recently looked at and reversed. It is labeled so that one end should go to the burner, the other to the gas valve. This has been backwards for years, but apparently did nothing to remedy the problem recently. Think It should be replaced?

        They said the next thing they will replace is the meter itself, which is outside. The furnace is in my basement, in an open corner,and has plenty of working room. The basement never gets below 50. It is currently -5 out and the basement temp, inches from the regulator is 55deg.

        ” it’s possible that freezing temps coming down the flue can freeze the fragile air switch diaphragm”
        I will have to bring this up to the repairman, who I suspect will be paying a visit tonight as the temps dip to the critical point. They may have checked this many times, but was not present or I don’t remember the conversation.

        Just watched a video on pressure switches. Questions do arise. How does the gas burner co. know what the correct pressure switch is for my furnace is? Replacement of such a switch with basic general purpose switches might cause fails when there is nothing wrong (yes?).


        Vent-pipe sagging….another possibility that may not have been looked into.

        Not sure I got an answer as to why a smaller diameter flume liner was placed inside my flume. Did you have a idea on this?

        Thanks for your time, Rick. I already think that with a couple of heads involved, the issue will be addressed more quickly.

        1. PatPro says:

          After reading the entire thread I suggest that you remove the tubing from the pressure switch after a successful run and see if you get any moisture. Even a few drips of water can refreeze from the downdraft causing an incorrect pressure reading. If you have 2 pieces of tubing connected to the pressure switch you are monitoring the pressure differential across the heat exchanger and you need to drain both of these. If this solves the problem, I would suggest installing a flue damper which opens only when the unit is operating. That will keep the cold air downdraft from entering the furnace.

          I would also check the rocker switches on the access panels. Less obvious but they are also wired in-line with the safeties.

          Finally I would check the flame sensor. If the ceramic is cracked they turn into a hit or miss type situation, which is exactly what you have described. At a minimum clean it.

          What i did not see you say is that you sat down in front of the furnace and watched the operation. That will give you more clues as to what is happening, or more importantly, what isn’t happening.

          Hope this helps a bit.

          1. Brian Bourassa says:

            I’m going to tel you what the problem was, in a nutshell. Poor record keeping and followup. 10+ years have gone by and they finally changed the gas regulator, which is located outside with the meter. The shear lack of communication and record keeping made it impossible for each repairman to reference what had or had not been done to fix the problem. In my case, the reference data needed to be carried over from year to year.

            I would have not expected this because I would have thought that once it froze(failed) it would not function
            a minute later after I pressed the reset switch.

            I have now been through at least 2 very cold spells, as low as -9, -14, and -17, all of which would have surely caused a repeat failure.

            I meant to ask the last time someone came over, about a “pressure” switch but forgot. This is an oil furnace with a natural gas retrofit burner attached, not sure where the switch is (on the burner?) not that I’m going to touch anything now as it is working.

            Thanks for all your time and advice.

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  46. jr says:

    rick i have a Armstrong ultra sx 80 the problem is the furnace calls for heat inducer fans comes on pilot light comes on then up burners come on then blower comes on furnace runs until heat temp is reached furnace shuts down blower keeps running for about 2 minitues then shuts off but then the inducer fan will come on and off for about 4 or 5 times until it shuts down . is this normal. thank you

  47. Terry says:

    Hi Rick. I have a gas furnace that has an intermittent problem. At times, when the thermostat calls for heat, it clicks, the Inducer draft/combustion blower starts, then the Hot Surface Igniter (HSI) glows Red Hot, but the gas never starts. Each time this happens, I can open the burner panel and moments later the gas flows and the burner ignites. The HSI glows red hot when this happens until I open the panel and it eventually opens the valve. Sometimes, the HSI must have been glowing for hours without an open gas valve. I checked all wiring connections and they seem ok. I have tapped on the side of the gas valve and nothing happens then, but always moments later after opening the panel, the burner finally lights. This does not always happen, but when it does, it is very annoying.

    Does this sound like a faulty gas valve? I ran temporary wires from the gas valve, outside the panel to an external volt meter, and so far after doing that, it has not failed. I intend to catch a failure, then observe whether or not the valve is getting 24 volts. I assume that if I see 24 volts during a failure, with a red hot HSI, the gas valve must be bad, correct?

  48. chris gund says:

    HI I have a furnace installed in 1994 hiel sp 80 with Honeywell smart valve 9500m ( apparently notorious for failure) I had to clean intermittent pilot and noticed on start up that gas burner is switching on before hsf ignitor is glowing and intermetent pilot is lit allowing some unburned gas into furnace . is this normal operation for this generation I am getting conflicting opinions on what the flame sensor is responsible for on this model

  49. Tim says:

    Hoping you can help out on this one. Goodman GMPN100-4 ReV B. Goes through the Cycle until HSI glows – No Gas. The SO and ROS’s, PS, etc appear to work. Gas Valve never opens. The transformer sends 24V to the 9-wire connector, but the circuit coming out and going to the SO and ROS’s that ends at the Gas Valve only gets 12V. It never jumps to 24V. The gas valve when jumped direct with 24V will open and flow gas.
    So it appears there is some logic that sends a 12v current through the safeties to make sure they are closed, then later jumps to 24V to open gas valve. I can’t figure out which sensor is not allowing the jump, or if the board is bad. It seems odd to me if the current was initially 24V, then it would be blowing in gas for a while before the HSI gets hot. I do not see a HSI sensor, that would confirm the HSI is hot before releasing the gas valve. Please advise

    1. Rick says:

      Hey Tim, sorry I didn’t see your message sooner. You might want to check with Goodman I know they had a recall on some of their control boards had an issue with my own furnace and a friends that had the same issue as you. I was able to get replacements for free but not sure how long they will be doing that for. Seems to be an issue with main control board. Good luck!

  50. Gord says:

    Hi Rick,

    I have found your information very helpful so far. My Bryant furnace is 18 years old. I turn the power on and the blower fan runs continuously. I have open flame roll out switch(FRS) fault showing. I have disconnected the wires from the FRS and left them open and powered on the furnace and I have installed a jumper between the wires and powered on but the fault still continues. I have checked for voltage at the terminal block and I get 3 volts AC. The induction motor does not come on at all but it does work when I put 120 volts directly to it which also proves that the switch works. The board has 27 volts AC coming into it. The thermostat terminal strip is showing 14 volts to ground. I had to clean the flame sensor a few weeks ago. I am starting to think that the main board needs replacing since the voltage is so low. What are your thoughts?

    1. Rick says:

      Hi Gord, Yeah sounds like an issue with the board and being that old that’s what I would bet. I would look on ebay for a replacement. Good luck!

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About Rick

I've been an HVAC/R Mechanic working in the Seattle area for over 15 years, specializing in the commercial service industry.. I’m also a Licensed Electrician & Gas Piping Mechanic and have numerous other trade related certifications. I’ve instructed at local trade schools and now continue teaching through this site.

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