I have a Lennox G24M3-75A-2 furnace (for my upstairs "zone"). This is an early model G24M with the single LED for diagnostics (ie, it is pre-SURELIGHT control board.) Now when the furnace comes on (from "call for heat"), all I am getting is room-temperature air coming out of the vents. The air keeps blowing indefinitely (even when set to auto). Inside the furnace unit, this is what I observe: When power is turned on (and thermostat is set such that call for heat will occur), I see the (one) diagnostic LED blinks constantly, then after about 45 seconds, the MAIN blower fan turns on, and the LED starts blinking 3 times (repeatedly). Note the small combustion blower does not come on at all. But the main blower/fan stays on. I do not hear the gas valve click at all, nor hear any sparking. I assume that those things do not occur if the pressure switch stays open (because it does not detect that the combustion blower is working correctly.) I don't hear any relays either (but I am not sure if they are loud enough to be heard). Any ideas? . I am hoping that maybe there is a reset button, or something else I can try myself. Can I try to measure the voltage somewhere to determine it the combustion fan is broken, or if my control board has gone bad? Or maybe it is something else? Any chance that replacing the batteries in my thermostat would make a difference?
According to the service documents for your model, 3 flashes on the original RAM control LED is combustion proving switch open. This doesn't mean the switch is necessarily bad. Several things can cause this error. Some are: failure of the inducer fan assembly, blockage of the flu or heat exchanger, debris in the inducer housing, leaking high temperature silicone tubing, restriction in the tubing or tubing connection to the switch or inducer, or a defective pressure switch.
CozyParts.com, Inc. is an Independent Lennox Dealer providing a national parts distribution resource for Lennox parts since 2002. The Cozy.Support team does not provide a diagnosis for users of this forum. We do, however, try to provide information that will help servicers arrive at their own diagnosis.
Here is an update, in case anyone else experiences similar symptoms. With the power off, using a screw driver, I moved the fan blades of the combustion/inducer blower (just a bit, less than 45 degrees). Then when I turned the power back on, it worked properly (ie, the furnace went thru the start-up sequence beginning with the combustion/inducer fan running). Thank god. Now I just hope that it KEEPS working.