A thermostat is a simple device that tells your air conditioner or heater to power up or off. Usually they do not fail, but in some rare cases they do.

Turn off the power to whatever it regulates, remove the thermostat cover to be able to see the wires.

You'll find two wires that you want to unscrew and take them off their terminals. Sometimes you see more than two wires, usually the red and white are the colors for power and heat. Air conditioners....., Red and Green.

Cross the two wires so as to short them and then urn the power on and see if the furnace blower goes on and the burner fires up. If it does, replace the thermostat, it's bad. If the burner doesn't ignite then your wiring may have a break in it somewhere, and you'll have to check for continuity.
Call a tech if your unsure where to go from there.

Learn more about this author, Richard Serra.
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