A gas furnace works with an electric igniter positioned in front of the fuel valve. If the igniter wears out or stops working for a different reason, the furnace will not light and the gas valve will not open. Checking the igniter takes only a few minutes with a multimeter to test the circuit and a visual inspection for signs of damage. The furnace must be at room temperature to conduct the diagnostic tests.
Go to the thermostat, and turn off the furnace at the thermostat. Typically, the thermostat is mounted on a wall in a central location in the home, such as a hallway.
Switch off the circuit breaker for the furnace. The breaker may be mounted on the furnace or inside the home's main circuit breaker box. Wait for the furnace to cool to room temperature.
Unscrew and remove the bolts that hold the service panel in place on the side of the furnace. That task requires a nut driver. Pull off the service panel, and set it out of the way.
Find the igniter in the igniter assembly. The igniter assembly is mounted on a metal bracket, which is typically V-shaped, in front of the gas port. The igniter has a flat metal tip surrounded by a ceramic housing. Two wires attach to the back of the igniter at the ceramic housing.
Shine a flashlight on the tip of the igniter. If its metal is warped, heavily scorched, chipped, cracked or broken in any spot, the igniter should be replaced.
Pull the plug on the end of the two wires attached to the igniter if the igniter appeared to the OK in the previous step. Needle-nose pliers may be needed for that task. Pulling the plug on the wire ends removes the wires from their socket inside the furnace.
Set a multimeter to the lowest resistance test, which is usually "Rx1" on the unit. Check the multimeter's user's manual if necessary.
Place the two multimeter probes on the two contacts at the tip of the power plug for the igniter. If the display on the multimeter registers infinity or holds steady at zero, then the igniter is defective and must be replaced.