First just want to say HVAC1000 and these forums have been very helpful to me in the past, in particular with a limit switch problem and I even got help getting a proper manual. I'm pretty sure my problem is the pressure switch so I'm hoping for a quick confirmation.

I've got a Goodman natural gas furnace (GMP075-3 Rev B) that's at least 10 years old. About a year ago I replaced a faulty limit switch (thanks HVAC1000). It has power and the fan blows when called by the thermostat, but there is no flame. As a test, I turned off power, waited a minute, then turned it back on. There is a sharp click and the fan starts immediately. Normally I would be able to see the flames on the burners in less than a minute, but there are no flames. To my understanding, there is no pilot for this furnace. The trouble light blinks 3 times and repeats. For this, the manual says check "vent or pressure switch vent blockage". Furnace was actively running when it failed in the middle of the night.

I removed both wires from the pressure switch, then turned the power on. After fan started, I jumpered the wires and in less than a minute the burners started. So this must be it, right? End of story? Anything else to test?

Please confirm for me that this must mean that the pressure switch is bad and needs replacing.

Here's my manual:
http://www.hvacpartsshop.com/Gmph075-4%20Series.Pdf

On Page 43 under "Pressure Switch Trip Points & Usage Chart" it shows part number B13701-33, if I'm reading it correctly. I can get it locally for about $30 (fair price), but it's not in stock and I'll have to wait until tomorrow for them to special order and rush ship it. Images I found online look right, but I'll have to buy it before I can put my hands on it. The local HVAC store says he can test it as one final confirmation, but I'll pretty much have to buy it first. So, any confirmation on the problem itself or the part number is greatly appreciated!

Seeing how the most quick I can get a pressure switch is tomorrow, would there be any harm in jumpering the wires of the pressure switch just for tonight? (See below.. I've ruled out other problems). If this just isn't safe, it's okay because I have a wood stove and enough wood to keep the house warm enough for a few days.

Things I tried before getting to this point:

- Made sure gas is on and thermostat is set to heat and calling for it.

- Removed the galvanized 3" exhaust pipe temporarily for test purposes. I ran furnace for a couple minutes but no change. Exiting air flow was strong and clearly not blocked.

- Removed the hose that goes from the pressure switch to the (what I'm going to call) fan chassis nipple and made certain it was clear. This is the only hose I could find.

- Jumpered the wires for the limit switch I replaced last year. It's still good.

Do not operate the furnace without the switch. This is a safety issue. I feel you are on the right track by replacing the defective switch so get it on order and replace it when it comes in. Now if you want to stand in front of the furnace acting as a manual safety control during its non-safe operation that will be your gamble.

Thanks man. I've got a wife and kids (and a wood stove) so last night I played it safe and didn't bypass the switch.

I'm surprised to tell you that it turns out the power switch was NOT the problem. I got some obscure advice from a different DIY forum which turned to be the solution. Apparently, the nipple that protrudes from the motor housing (that holds the hose going to the power switch) has a tendency to get rusty. I took a 1/16" drill bit and manually reamed out the nipple, ran the furnace for a bit to blow out the rust dust, reattached the hose and (original) PS, turned it back on and it worked! Apparently this is a common issue with this model so I taped the drill bit to the furnace as storage for future problems.

Thanks for your advice and concern. Have a good one.