The furnace thermostat controls the temperature in your home during the winter heating season. Thermostats normally deliver trouble-free service over many years of operation. But like any manufactured product, thermostats eventually fail and may leave you without heat. If your furnace fails to turn on, there are some simple things you can do to check out whether the thermostat is bad. The test procedure is the same for natural gas, oil, and propane furnace thermostats.  

1 Check that the thermostat is set for heat if it controls both the furnace and air conditioner.

 2 Check that the thermostat's set point is higher than the room temperature and that nothing is blowing hot air on the thermostat. No Matches Found. Please try your search again. 

 3 Check that the furnace's power switch and circuit breaker are turned on. If your furnace has a pilot light or condensate reservoir, check that the pilot is lit and the condensate reservoir is empty.

 4 Remove the thermostat cover to expose the inner workings and blow or brush any accumulated dust out of the thermostat. Sometimes, dust build-up interferes with correct operation of a thermostat. On most thermostats, you unclip or snap off the cover without using any tools. 

 5 Turn off electric power to the furnace if it still won't come on. 

 6 Undo the terminals for the two wires connecting the thermostat to the furnace and pull off the wires. Make a note of which wire goes to which terminal. In most thermostats, the wires are held by screw terminals. Unscrew the terminals with your screwdriver. Don’t allow the wires to drop into the wall cavity. If there are more than two wires, undo the red and white wires. These are the standard colors for the wires that activate the furnace.

 7 Twist the ends of the wires together. 

 8 Turn on the furnace power and watch the furnace. If the burners come on and the blower starts, then the thermostat is defective and must be replaced. If the furnace still doesn’t start, the problem is somewhere other than the thermostat. 

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