Furnace goes out when blower comes on...

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David Hepler

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Fenton, MI
First post but long time viewer.

My son called me to have a look at his furnace as it was getting cold in the house. He has a Goodman GMT0090-3A.

The furnace was lighting and then almost immediately the flame would go out. I removed power and had a look at everything and then cleaned the flame sensor as that seemed to be the most logical cause. Starting from cold the inducer will kick on, the ignitor glows, the gas valve opens, the flame ignites, almost exactly when the blower kicks on the gas valve closes and the flame goes out. It doesn't lock out and has no code. It will attempt to light again and it will ignite only to go out nearly as soon as it ignites. It will do this perpetually. Hes keeping warm only from that few seconds of fire every minute or so.

I tested the flame sensor with an ammeter per the service manual and found it to be in spec when flame was applied.

I suspected a fluttering pressure switch. I say fluttering since its closing when the inducer comes on... it must be working somewhat. I temporarily jumped the pressure switch and the furnace would stay lit and run as it should. Unfortunately, I let my cousin borrow my manometer so I didn't have it available to do a meaningful verification. My son was able to get a new pressure switch today and upon installing it the problem is unchanged.

I'm confused as to what could still be wrong that is "fixed" when jumping the pressure switch. Without a manometer I'm flying blind, I get that but I should still be able to sort some things. Im clearly getting a 0.6 or better draft or it wouldn't ignite at all. What would cause loss of draft, AFTER ignition? Like I say the burner will stay lit MUCH longer when the blower has not yet kicked on for a cold start. It goes out immediately when the blower activates. Subsequent attempts the blower is already running and on those attempts it lights only a couple seconds. Clearly draft is restored or it would not attempt ignition again.

I would suspect any number of other conditions including roll out switch or limit switch but WHY is the darn thing working as it should with the pressure switch jumped if those are the issue?

I did not leave the pressure switch jumped just to be clear... I know enough to be dangerous but this one is throwing me.
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