Regarding my Lennox. The machine will work in electric and AC mode and heat and cool. But in gas mode the Combustion fan never comes on and the pilot won't light. My question is how do I determine if the problem is the Combustion fan, or some switch, and which switch? Thanks.
If the pilot isn't lit, the fan control switch will not get a signal to turn on. The sequence goes something like: 1. Pilot-proving sensor okay (pilot on); 2. Thermostat demands heat; 3. Combustion fan on; 4. Combustion-air pressure detected; 5. Gas on; 6. Gas ignition detected; 7. Heat-exchanger temperature sensor lower-limit switch triggers air fan which runs; 8. Thermostat satisfied; 9. Gas valve turns off, 10. Air fan runs until lower-limit temperature is reached on the heat exchanger; 7 b. (If needed) Safety switch for heat exchanger upper-limit temperature cuts off gas valve. If this is a high-efficiency furnace, the exhaust-air fan sensor enters the equation with combustion-air sensor.
Is it that the pilot won't light or that it won't stay lit? there's a thermocouple which senses the pilot and keeps the valve to the pilot open. If the pilot proving thermocouple sensor is bad then the gas will never come on.
