Goodman GMH95 Intermittent Lockout

diesel600

Hello. I'm having an intermittent problem with my four year old Goodman GMH95 furnace going into lock out mode before coming up to the set temperature. It seems to usually happen when the furnace is running for an extended period of time, like in the morning when it comes on after being off for most of the night. I have my thermostat programmed not to call for heat overnight unless it gets down to 55 degrees F, so on cold days, it has to run for a while to bring the temp back up to 68 in the morning. I took the cover off and observed everything from a "cold start", and here is what happens:

1. Furnace starts up normally, ignites on first try, and runs fine usually for at least ten minutes or more. Flame looks good.
2. The flame will go out while the thermostat is still calling for heat. Induced draft blower and main blower remain on. The ignitor will glow, it sounds like the gas valve kicks open (or at least tries to), but it does not relight. This repeats a couple times and then it goes into lockout.
3. Once in lockout, if I reset the thermostat or the furnace power switch, it will start back up normally and the cycle repeats.
4. Once the house reaches the set temperature, it will usually maintain the temp without any issues, since the cycles are shorter.

The only stored trouble codes are the lockout code (1 blink).
I have so far replaced the flame sensor and cleaned out the condensate drain lines and pressure switch lines. Any ideas or advice are greatly appreciated.

-Scot

Doc Holliday

If it's igniting, if the ignitor glows, then it's not a pressure switch issue. The pressure switch needs to close for the ignitor to light up and you're getting there.

Might be a gas valve (need to check voltage to it as the ignitor glows) or the control board may be faulty.

__________________
Thanks.

Mule98

Check your flame detector.Had a Goodman and this was the most common problem

diesel600

Thanks, Doc. I will get out the multimeter after work today and see what I can find out. I was thinking possibly the gas valve as well, due to the lack of trouble codes for anything else, but it just seems strange to me that it never has a problem firing up initially, and only cuts out after running for quite a while.

diesel600

Already replaced the flame sensor, Mule98, as stated in the initial post. When it had a bad flame sensor, it would relight every time and then go right out. This time, when it tries to relight, there is never a flame at all.