I have a 6 years old Goodman furnace [GMH950703BXAG]. I keep getting the code 3 (3 FLASHES - Pressure Switch Stuck Closed). The exhaust motor kicks on, runs smoothly (no odd noises), but the furnace never gets flame and starts the blower motor. The pressure switch is labeled -0.95. Using a manometer, I measure -0.74 with the exhaust motor only running (no flame), if i bypass the pressure switch and light up the furnace, I measure -1.15 in-H20, the same pressure is measure on either side of the heater plate. When I bypassed the switch i watched the flames as the blower comes on to check for disturbances in the flame, no disturbance at all (to check for a potential heat exchanger crack). I checked the intake and exhaust pipes, they were clean. I cleaned the hose going to the pressure switch, I removed and bleach-cleaned the water container on the side of the furnace that holds the water from condensation. I removed, inspected the heather plate, no dirt, no cracks. I removed the turbulators from the secondary heat exchanger and cleaned them with soap and water (there was no deposits on them, just slight discoloration). I added high temp silicone when putting the heather plate back on, no difference in results after installation. To be safe on the safe side, I replaced both the pressure switch and the exhaust motor, no changes in behavior.
Anyone have an idea of what else I can do? I have run out of ideas, there is a long wait until I can get a someone to come in.

 

You should almost never bypass the pressure switch with the gas on even if it's for a short period of time - the one exception is if you're monitoring for and confirmed proper draft with a manometer.

Your reading confirms it's not seeing a proper draft.

Did you make sure where the pressure switch hoses are connected there are no blockages? Not talking about the hoses themselves.

The "water container" is a trap - did you remember to reprime it? It most have water in it to function.

 

Thanks for the reply, I only bypassed the switch for troubleshooting purposes.

There are no blockages going to the pressure switch, the heater plate is clean, with no obstructions in the port.

I did re-prime the trap after cleaning it to ensure proper draft. The thing that I can't answer is why the negative pressure increases within one second of the flames igniting and stays within a good pressure range until the flames turn off.

 

The burners impact the draft - there's a venturi effect going on.

When there is not a sufficient draft before the burners come on, it's not safe to fire the burners by bypassing the switch even for troubleshooting purposes. Can cause an explosion worst case, best case bad combustion, massive amounts of co in the exhaust, possible sooting of heat exchangers.

Shut the gas before bypassing pressure switch - the ignitor coming on confirms it's the switch not closing. With a meter, there's no reason to bypass stuff though.

You should see if anything has gotten into the exhaust pipe - may look clear outside but you never know.

 

The ignitor does not turn on because the pressure is too low with the exhaust motor only and no flames (only sucks -0.74 draft, instead of -0.95). I also removed the exhaust pipe altogether without flame and pressure switch measured the same -0.74, so exhaust pipe has no effect on draft.

 

That's very odd.

You said the error code = "pressure switch stuck closed" -> the normal error for this should be "pressure switch stuck open".

 

Goodman three flash fault code is • Pressure switch circuit not closed.

 

You are correct, I screwed up the code in the original statement, it should read 3 flashes - "pressure switch not closed"