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We have a Goodman model AMS81005CXAC furnace with a Lennox AC on top of it. PCB is *134.

About a week ago, when coming back from a vacation when it was set to 55 for two weeks, it was starting/stopping frequently. Turning it on and off seemed to fix it. It is now doing it again (video below) and can't seem to get it going. Furnace filter is clear. Furnace is about 7 years old, was done under a home warranty when we bought it. I'm pretty handy - is this something I can reasonably fix?

Video is here: https://www.youtube.com/shorts/RPlvrumVx14

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  • Can you post a photo of the furnace's wiring diagram please? Commented Jan 4, 2023 at 12:46
  • I believe this is the correct one from their PDF: i.imgur.com/Zv6bI6f.png
    – James F
    Commented Jan 4, 2023 at 12:55
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    @ThreePhaseEel This is their online manual that appears to be the same as my physical manual - the last page (70) with board 134 is what I have: hvacdirect.com/hvac/pdf/GDS8-Service.pdf
    – James F
    Commented Jan 4, 2023 at 13:06
  • I've done my best to interpret the very dark picture of the model number sticker. Instead of leaving it to us to do so, though, it would make everyone's life much easier if you would put the model number of the furnace into the question yourself. Please make sure that the model # I somewhat-guessed at is correct so people will be able to help you.
    – FreeMan
    Commented Jan 4, 2023 at 13:19
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    When you say start and stop, what does that mean? Does the initial fan turn on? Does the gas ignite? If the gas ignites, does the sparker continue to click or does it stop? Also, as others mentioned the PCBs often have a diagnostic LED or something like that, which usually pinpoints the issue.
    – Smith
    Commented Jan 4, 2023 at 14:30

1 Answer 1

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The brief running of the inducer suggests that the control is unhappy with one of its inputs (safety switches). Turn off the power to the furnace and with an ohm meter/continuity checker test the following one at a time by disconnecting either one of the leads and then measuring across the two terminals of the device:

  • burner high limit switch (circled in green, mounted between the burner ports and the bottom of the collector box)
  • flame roll-out switches (circled in red, one at the left and one at the right of the burner manifold)
  • blower compartment high limit switch (circled in blue, mounted above the control board)

switch locations

The two high limit switches are auto-resetting, so if the furnace is cool and either of those is open (infinite resistance) then it has failed and needs to be replaced. The flame roll-out switches are manual-resetting; if either of those is open then reset it by clicking the little button between the terminals and test again. None of these switches should trip in normal operation, but particularly if a flame roll-out switch trips, somebody needs to figure out why it's happening.

There should also be a pressure switch. It's not visible in any of your photos but I think the two yellow wires going to the upper-right corner of the photo lead to it. Check whether that switch is open or closed. I'd expect it to be open. If it isn't, then try disconnecting one lead and powering up the furnace. If this leads to the inducer running longer than the half-second or so shown in your video, immediately reconnect the lead to the pressure switch and observe what happens next. (This would be a sign that the pressure switch has failed closed.)

As an aside, there's a lot of corrosion at the bottom of the collector box cover and a fair bit at the inducer output too. Things are getting wet that shouldn't be wet. The corrosion at the inducer output would have to be caused by water coming down the flue; the stuff at the collector box cover could be from that or possibly condensate from the air conditioner, a humidifier problem, etc.

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  • Thanks! I will check this when I get home and see what happens. Regarding your comments "circled in [color]", I'm not sure if an image didn't upload, but I should be able to find them and test. Yeah, I noticed the rust. The AC was put in by a different company at a later date, and there is a (currently non-running) humidifier.... I'll check both, thanks.
    – James F
    Commented Jan 4, 2023 at 17:48
  • @JamesF oops! I had failed to actually paste the image in. :-(
    – Greg Hill
    Commented Jan 4, 2023 at 18:20
  • Well, I started trouble shooting, then at one point I took off the thermostat face plate to check something and since then (after putting it back on), it's been working. I found a reference online where someone said a failing faceplate could cause it. Unfortunately, at this point, I can no longer recreate the issue, so I'll have to try when it happens again. Thanks so much for your help!
    – James F
    Commented Jan 5, 2023 at 14:23

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