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mranswer, HVAC Mechanic
Category: HVAC
Satisfied Customers: 6160
Experience:  20 years experience running my own business
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Comfortmaker Furnace model GNJ075N12A1 burner wont stay lit.

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Comfortmaker Furnace model GNJ075N12A1 burner won't stay lit. Checked all connections and just replaced the igniter and thermostat. Everything kicks on, even the burner once in awhile, but doesn't stay lit for long. Most times though, it doesn't kick on. Furnace was installed in 1995. I replaced the gas valve and flame sensor in 2004. The relay from the thermostat on outside of furnace if part 4000-01A19X82. The board is White Rodgers 50A55-438. The relay (inside furnace) is 4000-01E07AX191. I sure hope it's not another gas valve as I just cannot afford that right now. Thanks in advance for you immediate assistance.

I would first eliminate your wall thermostat as the issue only if the inducer will not even start up on the call for heat. Jump out the red and white wires that connect to it either at the t/stat or at the furnace end if the inducer does not run as explained. The furnace will start up. If it stays running then the t/stat is bad. If the flame goes out after starting then you have flame sensing issue, so that sensor could well be out again as a gas valve is typically going to open or not.

Customer: replied 4 years ago.

Where do I find the flame sensor and what does it look like? Not the igniter as I already replaced that.

The flame sensor is part of the ignitor assembly -you said you replaced it in your post. Make sure it is clean and in the flame. Use some steel wool to lightly buff it.

Customer: replied 4 years ago.

Cleaned sensor. Turned furnace back on, flames lit and ran for several minutes. Flames shut off, but fans keep running and blowing the cold air continuously. If I flip the main switch on the side of the furnace off and back on, the system runs normal for several minutes. Again, the flame goes off and fans continue to run. The blower does not shut off for quite awhile, if at all.

This may be a flame sensing issue or a problem within the mechanical part of the valve. The gas valve provides the flame sensing on this unit, so with the problem being the flame sensing then it looks like you need a new valve regardless of what is internally wrong here. The valve is probably pretty costly and there may be a retrofit kit made that you can install as this is getting old. You should have 24 volts being sent to that valve on the call for heat between terminals 1 and 3 on the 4 wire connector per the schematic that I see. This would prove that the board is fine.

Customer: replied 4 years ago.

Is there any way to take the valve off and test it?

No, you can only make the voltage test as I described to eliminate anything external to the valve.
Customer: replied 4 years ago.

OK, we'll go from here. Thanks for your help.

Thanks. Good luck here.
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