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Goodman GMP125-5 furnace burners won't fire

I have a Goodman GMP125-5 furnace that just quit working a few days ago. The furnace is laying horizontal up in the attic crawl space. I can hear it trying to start but after a while it shuts back off. Thinking it was the igniter I took the doors off and tested it with a meter. I got 35 ohms on it and then tried to restart and got three blinks on the error code led. This indicates a bad pressure switch I believe. I have ordered a new one but in the mean time tried drawing or blowing air through the air hose and could only get a very little air to move with a lot of pressure. Is there something else I should be looking at to confirm if something else might be bad in addition to or instead of the switch?

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  • timbeaty Mar 18, 2008

    Thanks for the quick response. I have further tested the pressure switch by checking resistance while at idle, it was open as it should have been. Applied a light suction to the switch, I could hear the switch operate, and resistance fell to almost 0 ohms. This tells me that either the switch is okay or maybe has a sight leak in the diaphragm that I overcame with all of my hot air. A new switch should be here tomorrow or the next day and I will replace regardless. I could not blow or **** any air to/from the venter blower assembly. Putting a jumper between the contacts of the pressure switch does allow the burners to light normally. I do not have pressure gages except for automotive ones and I am not sure that they would measure the vacuum that should be coming from the assembly. I believe that the circuit board automatically resets itself after each failed attempt. I also believe that the 3 blink error code "pressure switch failed" would also show up if I have a blocked vent or something wrong in the venter blower assembly since it is the vacuum from that assembly that closes the switch. Is there a way to test that assembly? I am going to try and get a air hose up there to blow the dust off of the fan on the assembly. It will take several days to get a new assembly here if I need one but it is a lot more expensive than the switch so I didn't want to order one before I ran some tests that might confirm that it was bad.


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Try to bypass the connection of the pressure switch to confirm if the pressure switch is really bad, and reset the unit. Monitor your pressure to be sure that its normal and not the cause of tripping your pressure switch.

Posted on Mar 18, 2008

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