I have a 5 year old Lennox G40UH gas furnace. The furnace runs for short periods (2-3 minutes) 10-15 times per hour. This happens even when the house temp is above the thermostat setting. I have replaced the thermostat but the problem continues. I have checked the diagnostic lights before, during and after these short cycles and they always flash normal. If the thermostat is turned up a couple of degrees, the furnace runs normally until the desired temp is reached. Only then does it seem to start this unnecessary cycling. Any ideas? Thanks Paul

Use a multimeter to see if the thermostat is shutting off the furnace.


Measure the AC voltage between the C and W terminals. When you have 24 VAC, the thermostat is switched on and the furnace should continue to run until switched off.

Check to see if that voltage goes to zero when the burners are shutting off. If so, you have a thermostat issue. The thermostat probably doesn't need replacement --- but check to see if that's the problem first.