I've got a 6-year old Lennox, G40UH-48C. It's controlled with a Honeywell programmable thermostat. The last couple days the furnace won't ignite when the stat calls for heat unless I cycle the power, then it ignites and heats to set temp. It may even cycle for a few hours but then the house gets cold again. Stat says "system on" but the LED's at the furnace intermittently blink, "won't ignite". If I simply cycle the power, it ignites just fine.
Not sure what to investigate. I don't know what I should be reading with my VOM at the thermostat either.
I cleaned the flame sensor - PROBLEM SOLVED! Hasn't acted up since and I didn't clean anything else. I can't say that the sensor looked like it needed cleaning but I've never done it and obviously that's what was causing the furnace to drop out into a failure mode.
Thanks for your time!