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Hi all - I have a Lennox G40UH-series gas furnace that is behaving erratically. I have it connected to a programmable Honeywell thermostat that calls for heat in the morning and AC during the day (in Florida). One morning last week the heat didn't come on: no burners, no blower, LEDs on circuit board both out. When I got home from work, the AC was running fine. Next morning, the blower came on but was blowing cold air; the furnace was not lit. The unit wouldn't shut off from thermostat, so I did a hard reset from the breaker box. Still no lights on board. I checked voltages on both sides of the transformer and at the thermostat; all good. Later that day, both the AC and heat fired up no problem. Next morning, same deal, nothing was working again. I thought it might be the ignition board, but I decided to have a tech come out yesterday. He replaced a bad float switch and the run capacitor on the blower motor and had the AC running fine with both lights green on the circuit board. I thought we were good. This morning, however, back to the same: no lights, no blower. Already checked the thermostat. Any ideas?
 

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probably a bad circuit board. when idle there should still be green lights on indicating power is on to the board. depending on the model they may flash slowly (hearbeat) and faster when calling for heat. you had power to the transformer so there was power to the board and with no lights that means a bad board. cozyparts.com is a good Lennox parts place.
 
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