Problem scenario
A furnace keeps getting hot exceeding the desired temperature of the thermostat. It never stops and is creating a great deal of heat. What could be wrong?
Possible Solution #1
It could be a thermostat issue. Can you replace the thermostat or remove it from the wall to see if that stops the heater? If it keeps getting hot, see the other options below.
Possible Solution #2
The limit switch could have an issue. A professional could turn off the power and check the limit switch with a multimeter. (This would be possible if removing the thermostat does not shut off the heater.)
Possible Solution #3
A sequencer or the entire control board could have an issue. A furnace expert could replace this. (This would be possible if removing the thermostat does not shut off the heater.)
Possible Solution #4
Is the furnace relatively new but the house itself is much older than the furnace? If this is your situation, the wires in the wall may be old. A wire in the walls has shorted, or it is starting to short. This can cause intermittent problems of overheating from the furnace (exceeding the temperature of the thermostat). A furnace installer with electrical knowledge could fix this. (This scenario could happen if removing the thermostat does not shut off the heater.)