Lennox G50 UH Furance won't stay lit.

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Old 11-07-11, 05:00 PM
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Lennox G50 UH Furance won't stay lit.

Greetings,

I have a Lennox G50UH-24A-045 furnace. When we lit it for the first time this season (maybe 2, 2.5 weeks ago), it would not stay on. Like many others have reported, it burned for a few seconds (I timed it between 2 and 30 seconds), then the flame shut off. After 5 times the WatchGuard kicked in and it shut down completely.

After reading the message boards here and elsewhere, I cleaned the flame sensor and retested, with no change. I ordered a new flame sensor, but after installing that the problem persisted. I tested the output of the flame sensor, and was getting about 2.0 microamps.

At this point, I called the installer. He came out, tested the same things I had done with the multimeter, and then said I needed a new circuit board, which he would be glad to do for $595. Instead, I ordered a new one online for ~$120.

Well, today the new one came. After installing, I'm still having the same issue as before. Did I just get a bad replacement board? Or is there anything else that could possibly be wrong?
 
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Old 11-08-11, 11:41 AM
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Make sure the sensor isn't touching any of the burners or other metal components within the furnace. It will ground it out and cause it to not work.
 
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Old 11-08-11, 02:03 PM
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Describe in detail the sequence of events you observe when you turn up the thermostat.

Identify the terminal on the circuit board that the pressure switch energizes when it cloases. Measure that voltage to see if the 24 VAC from the pressure switch is shutting off, which will shut off the ignition sequence.

Usually a circuit board will leave the burners down to a DC current of about half a microamp. So at 2 microamps I would usually expect the burners to stay lit.

A good microamp read is usually about 5 ua.
 
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Old 11-08-11, 04:16 PM
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Thanks for your replies!

kyoung22 - I tested the flame sensor; it is positioned correctly, and is not grounded out.

SeattlePioneer - I will test your suggestion tomorrow after I replace my multimeter, thanks to my hyperactive dog. Don't ask.
 
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Old 11-11-11, 05:01 PM
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SeattlePioneer - It was the pressure sensor, but there was nothing wrong with it. The problem was the 5 dead birds in the flue, blocking the air flow.

Now that those are cleared out, it's working like a champ. Now I just need to figure out how to get on top of the chimney and put some chicken wire over it.

Thanks for your help, everyone.
 
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