My gas furnace properly lights and runs for 16-20 seconds and then the gas shuts off. Here's what I know...

The flu is clear and the inducer blower continues to run. The pressure switch appears to work properly (it clicks when I suck on the blower end). The two leads on the p/s register 26VAC when the furnace is on and calling for heat. Within a couple of seconds, that voltage goes to zero (as the p/s is detecting good air flow); the voltage returns to 26VAC when the gas is cutoff. The circuit board LED then blinks 3 times.

I did have a seized inducer motor a few days ago. I found it humming and manually gave it a goose with my hand to get it going. It works fine now but definitely needs to be replaced because it's making more noise than it should and not turning as freely as I believe it should.

My thought is that maybe the inducer blower isn't pulling an adequate amount of air and kicks off the p/s or... the p/s is bad. I suppose the circuit board could also need to be replaced but I don't want to order them all but since I need to mail order them, I also don't want this project to drag on another week or two as I try replacing different parts.

I'm leaning towards ordering a new inducer motor since that will need to be replaced anyhow.

Any help would be greatly appreciated. Thanks.

Have you tried cleaning the flame sensor?

Yes, it's been cleaned. Tried connecting that lead to ground before I found out what 3 blinks meant (no diagnostic table on my door).

The circuit board diagnostics are not the most reliable on Goodmans. I have seen quite a few cases of faulty pressure switches that the board fails to identify correctly, even giving an unrelated fault code. If the switch contacts become contaminated or oxidized, they won't support the circuit current when the gas control is powered, but maintain enough current to trick the board when the trial ends. I detected this by jumping the pressure switch wires together at the appropriate time.

Thanks for the input KC13. Do you recommend trying to hot-wire the p/s as a quick test?

I tried that by connecting the two leads but then I got 1 blink from the board, which means the p/s is stuck closed. I see you mentioned you timed connecting them. I'm assuming the board sends 24VAC on one of those wires and tests for the return signal on the other.

I'm a bit confused because I would expect the voltage readings I posted in my original post to be just the opposite (i.e. 0 volts with p/s open, 24v with it closed (while inducer running), and then 0v when it re-opens). What am I missing? Isn't the p/s default state open (inducer not running)?

What steps do you recommend next?