[T0133]

Heil furnace: model N9MP2100J20A3, serial L0241 30932
Programmable thermostat set to 65 degrees overnight, 68 degrees during the day. Woke one morning to indoor temp at 80 degrees. Set thermostat to 'off', burners did not turn off. Had to shut off main power switch to furnace to turn off burners.
Called our regular HVAC service company. Furnace worked okay for them, but technician saw some dark areas around a resistor on the main circuit board and suggested replacing it. Did not see any sign of problem with the thermostat. Replaced the circuit board and it worked okay for a few weeks, then started doing the same thing. Replaced thermostat, but did not fix the problem.
Regardless of position of switches (on/heat/cool, fan on or auto) the combustion intake fan will activate and burners will ignite when the main power is turned on.
The only recent maintenance done is replacement of the fan motor capacitor because the maintenance tech said it had lost 10% of its capacity. I ordered one with the same micro farad rating and voltage rating as the one on the furnace. I received one with the proper micro farad rating, but with a higher voltage rating. I was not concerned with the voltage, since I had read that you could go with a higher voltage, but not a lower voltage. This told me that the rating was a capacity of voltage that the capacitor would hold, not what it would create (I don't believe a capacitor creates voltage anyway). Could the higher voltage capacitor be causing my problem?

[mygirlsdad77]

Capacitor isn't your problem. I would start by unhooking the themostat wires from the control board on the furnace. Does the draft inducer still run then? If not, then there is a short in the stat wires between the stat and furnace. If yes, you have another bad control board, and need to find what is taking them out.

[T0133]

I unhooked the wires from the thermostat, and the draft inducer still runs. By your comment above, that points to the control board. However, this is an intermittant problem. Most of the time, the problem still exists, but since trying your troubleshooting suggestion, it has operated correctly and responded properly to the input from the thermostat. Any other suggestions, or does this still point to a control board issue?