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Old 11-12-2011, 09:18 AM   #1
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Lennox 80mgf problems


My lennox was installed in 1999. New board put on in 2002. New flame sensor 2008. Another new board Feb 2011. Now it is not completing a cycle. The flame sensor glows, flames shoot, kicks off. it will do this for an hour until it locks down. The repairman tried an air switch and that didn't work either. I can manually start it.

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Old 11-12-2011, 09:22 AM   #2
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flame sensor and the face of the burner it sits in front of need cleaning with fine emory paper once a year. should have a NEW diagnostic code sticker that came with the new board attached to the plate it sits on or side of furnace. read the flashing lites and get the code and post it here.

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Old 11-12-2011, 09:45 AM   #3
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Lennox 80mgf problems


When I manually start it the green and red flash fast in heat mode. When the cycle ends, only the green flashes slow.
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Old 11-12-2011, 09:46 AM   #4
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Lennox 80mgf problems


The slow green only light means flame sensed without valve energized.
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Old 11-12-2011, 09:49 AM   #5
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probably the porcelain on the flame sensor is cracked or the wire to the sensor is shorted to ground. or the tip of the sensor is hitting the burner. are you SURE you are reading that code properly. if it is for real then the burner should not fire up. board does a self check and checks for that problem B4 it fires up.
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Old 11-12-2011, 09:53 AM   #6
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Lennox 80mgf problems


Thanks, I will check it out.
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Old 11-12-2011, 09:54 AM   #7
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Lennox 80mgf problems


reread my post, I added to it.
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Old 11-12-2011, 10:06 AM   #8
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The codes are on a sheet the repairman gave me when they put the new board on in February. I am unplugging and replugging to get the furnace to kick on so possibly bypassing the board?
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Old 11-12-2011, 10:10 AM   #9
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No way to bypass the board. Usually the board checks for that problem and the pressure switches B4 it tries to start. Easy to mix up those codes. There should be in small print diag 1 and diag 2 above the leds. check with a magnifying glass.
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Old 11-12-2011, 10:38 AM   #10
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The list of codes on the furnace read this for the green slow flashing light:
pressure switch open or has opened 5 times during a single call for heat or blocked inlet exhaust vent or condensate line blocked or pressure switch closed prior to activation of combustion air blower.
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Old 11-12-2011, 10:50 AM   #11
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Lennox 80mgf problems


AHA. probably you are not getting enough draft thru the furnace to close the pressure switch or keep it closed. at that age the ventor fan may be slowing down and they had problems with paint flaking off inside its housing and plugging the port for the pressure switch. tech needs to check the draft and chimney for blockage. a very nasty difficult job to change that fan as the screws are inside the collector box so you need an experienced lennox tech or someone who knows how. take the hose off the spud where it attaches to the ventor fan and gently poke it thru with a paper clip or small drill bit.
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Old 11-12-2011, 11:51 AM   #12
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My computer froze up and it has taken me an hour to get back online. That makes sense. I did hear what sounded like something falling in my flu a couple of weeks ago. Could that be an issue?

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Old 11-12-2011, 12:03 PM   #13
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Yep. And that can be VERY dangerous if you have a gas water heater. Fumes and CO can spill into your house so you want to get it checked ASAP, like TODAY!

Even w/o a water heater it is dangerous as the furnace can get a flame rollout and fire.
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Old 11-12-2011, 12:05 PM   #14
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Thanks!
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Old 11-12-2011, 12:07 PM   #15
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reread my post as I added to it.

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