As you turn on a gas furnace thermostat, it sends a signal to light the burners and turn on the blower. This exact same signal sends power to this ignitor to help light the gasoline in the burners. If your furnace makes clicking noises as it lights, it is equipped with a spark ignitor. The clicking noise you hear is the electrical current creating a spark to light the pilot. Once the pilot lights, then it lights the gas supplied to the burner in the manifold. When the spark ignitor fails to begin a spark, then the furnace cannot produce heat.
Shut off the electrical power to your gas furnace. Depending on the installer and the local codes at the time of installation, you might have a disconnect switch next to the furnace. Turn off the breaker to the chimney at your main electric panel if you do not locate a disconnect switch.
Remove the burner compartment door in the front of your furnace. Start looking for the gas control valve inside the compartment that provides gas to the burners. Depending on the plan of your furnace, the burners might be on very top or the base of the burner port.
Find the little, metal gas pilot tube connected to the gas control valve. Trace the line till it terminates at one of these gas burners. The gas line connects to a metal burner assembly bracket that holds the pilot tube and also the spark ignitor. Some burner assemblies can hold a flame sensor as well. The spark ignitor looks like a 1-inch long metal probe attached to one end of a solid ceramic tube with an electrical wire attached to the other end of the tube. If your furnace needed a hot-surface ignitor, it might appear like a flat piece of metal attached to one side of a square piece of ceramic using 2 wires attached to another side of the ceramic. Both ignitors are attached at the burners, but the hot-surface ignitor doesn’t utilize a pilot or have a flame sensor attached to it.