A furnace requires proper gas flow to the burner assembly as well as electricity to the thermostat to actuate the heater. Usually, when you set your thermostat to "Auto," or automatic, the burner will ignite when the temperature falls below the temperature you set on the thermostat. What happens when the burner won't ignite and the fan won't blow warm air into your house? There are several causes for the heater not activating, and you can troubleshoot the problem yourself before calling in a technician.
Check the circuit breaker controlling the thermostat to make sure that it is on. If the breaker has tripped, turn it back on, then switch the thermostat to "On." If the breaker trips again, there is a short circuit in the system.
Turn the circuit breaker off. Remove the screws from the thermostat cover by turning the screws counterclockwise. Pull the thermostat cover straight off.
Push the anticipator gently to the left and right. The anticipator is the small metal tab in front of the temperature numbers that points to the current room temperature.
Lightly dust the anticipator and all metal contacts with a small paintbrush. Accumulation of dust may not allow the anticipator to move and turn the thermostat on.
Tighten the screws on the contacts that hold the wiring in place. A thermostat requires the wiring to touch all of the contacts to turn on. Replace any broken wires to the thermostat.
Replace the cover on the thermostat. Replace the screws and tighten them clockwise. Turn the breaker on and test the thermostat. If everything works, switch the thermostat back to "Auto."
Check the gas connection to the furnace. Turn the main gas valve completely counterclockwise to open it and allow gas to the furnace.
Replace the air filter if it is dirty. A clogged air filter will not allow the furnace to operate.
Remove the screws from the furnace cover to gain access to the pilot light. Light the flame with a kitchen match if the pilot light has blown out.
Replace the furnace cover. Tighten the screws by turning them clockwise with a screwdriver.