Hello, I have a 20 yr old furnace Ducane model # CMPE075U3 . It has been fairly reliable with a couple of glitches such as Exhaust gas blower motor needing replacement, Ignitor replacement 7 yrs ago and a capacitor for the blower. Other than that has been chuggin' along (knock on wood). Couple of nights ago it seemed to have shutdown ; I woke up in the morning to see the thermostat actual temp at 57 deg. F ; overnight setting was left at 64 deg. F. Went down in the basement to hear the exhaust gas fan running ; and the the ignitor was on (orange flame) but didn't trigger the pilot nor the burners (thus the blower was not activated). I turned the power off to the furnace (i.e flicked the power off to the furnace) ; left it alone for a min ; hit power back on and then it all fired up normal ; we got the first floor back to thermostat temp (64 deg. F). Upon reaching set temp ; it shut off the furnace as it should, BUT, never ignited the burners upon falling temp. Once again turning off the power to the furnace for a min ; she lights up, blower works well, we get heat. I noticed the ignitor / flame sensor (Honeywell part # Q3400A1008 ) didn't seem to pass gas through it's pilot section unless I go through the process of turning the furnace OFF completely (i.e the flick of the switch as I mentioned above). I am "assuming" due to 7 yrs of use in the New England winters that perhaps the ignitor needs replacement, maybe the flame sensor is not getting hot enough??...still can't put a finger on why shutting off for a min then turning on the furnace makes everything come back to normal. Thank you in advance for your response.

Hi, I have a tempstar furnace with a Honeywell S8610U ignition module and a Honeywell q3451 style burner pilot (it has a single spark/flame rod, so there is only one connection to the spark on the ignition module). When calling for heat with a nest thermostat the exhaust fan turns on, pilot lights and furnace burners light up. After a little while the fan kicks in. This seems to tell me that the limit switch is operating correctly as it is turning on the fan once furnace warms up. All good so far. After about two minutes the burners shut off, and power is cut to the thermostat. The fans continue to run for another minute or two (I am assuming this is due to the limit switch not reaching its cool limit yet). After approximately 30 seconds, once the power is restored to the thermostat, the pilot will light up again and repeat the cycle. Sometimes I have to adjust the temperature on the thermostat in order to force the heat to come on again. The ignition module does not report any specific error codes, it seems to only report the flame current (which seems to fluctuate between 4μAand 7μA. If anyone has suggestions what could be causing the temporary shutdown, or what should be looked at it would be greatly appreciated. thank you