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Old 07-25-2020, 04:40 PM   #1
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Lennox G40uh Furnace dies sometimes


Please help me diagnose my furnace problems.

Over the last 5 years my furnace has gone dead about half a dozen times and resurrected after a few hours. Itís happened twice in the last week.

The 2 diagnostic LEDs are dead. I measured the voltage at the 24V transformer. Itís generating 27V just fine. The fuse is ok. The red wire leading to the thermostat has 0V. This kills my AC also which is what I need these days.

My guess is the Surelight board has a problem. I saw some corrosion/deposits on the big IC on it. I cleaned it with 91% alcohol but it made no difference.

The furnace is a Lennox G40UH-48B-090X-03

Can you tell me which Surelight board I need. Does anyone have a schematic of the board and any tips on what common failures on it are, if I try to fix the board instead of replacing.
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Old 07-25-2020, 05:14 PM   #2
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Re: Lennox G40uh Furnace dies sometimes


Try cozyparts.com for Lennox parts

We don't repair those boards so there are no common failures.
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Old 08-11-2020, 03:12 PM   #3
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Re: Lennox G40uh Furnace dies sometimes


I got no help from cozyparts - they didn't even let me register.

I did fix the problem though.

It turned out that the 12-wire connector through which 24V was going from the transformer to the Surelight board had a bad solder joint.

Instead of taking out the board and fixing the solder, I ran another wire from the transformer to the fuse. It's held on both ends by alligator clips. Seemed sturdy enough but if it ever fails I'll resolder it.

Tracking down the problem took some work. I found an ebay seller of the same board. It had pictures of the front and back. I used them to figure out the circuit diagram and traced the power flowing in. It didn't take much work after that - it turned out the issue was near the beginning of that flow.
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Old 08-11-2020, 04:04 PM   #4
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Re: Lennox G40uh Furnace dies sometimes


Make sure the jumper wire does not fall off and short to ground B4 the fuse or you will blow the transformer.

Intermittent problems are the worst. Glad you found it.
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furnace , intermittent , lennox , surelight


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