Your home thermostat switches the air-conditioning or furnace unit on and off according to a predetermined temperature setting. If the furnace is not starting when it reaches the temperature, it is necessary to troubleshoot where the fault lies. A good place to start is at the thermostat. A quick way to determine whether the thermostat is faulty is to test it with a jumper wire, which creates a jump from one terminal to another to automatically start a HVAC unit.
Remove the cover from the thermostat. Depending upon the make and model, the cover either pulls straight off or slides up from the base.
Turn the screws securing the base of the thermostat to the wall counterclockwise with a Phillips-head screwdriver. Pull the thermostat base away from the wall just far enough that you can see the terminal screws and wires on the back. Do not disconnect any wires.
Strip 1/4-inch of insulation from each end of the 6-inch piece of low voltage wire with a pair of wire strippers.
Touch one end of the jumper wire to the terminal with an "R" stamp. The red furnace wire attaches to this terminal. Place the other end of the wire to the terminal with a "W" stamp. The furnace or air-conditioning unit should start.
Lift the ends of the jumper wire and move one end to the "Y" terminal and the opposite end of the jumper wire to the "G" terminal. The furnace fan should start operating. If the furnace and the fan start using the jumper wire, the thermostat requires replacement. If they do not, there is a problem within the furnace.