If your furnace is working intermittently it is an indication of an electrical problem, and a common cause is the igniter. As the temperature drops, the thermostat sends a signal to the furnace's circuit board, which sends a signal to a relay switch to turn on the igniter. A simple way to test a furnace igniter is to determine whether or not current is reaching it. If all circuits are working and the proper amount of current is reaching the igniter, than the igniter is faulty.

You can test a furnace igniter with a multimeter.

Step 1

Turn off the power to the furnace and disconnect the igniter by pulling apart the plug-in connector that attaches the furnace leads to the igniter.

Step 2

Attach the meter to the furnace lead terminals and set the meter to 110 volts. Use the alligator clips to hold the meter probes in place if necessary, being sure they do not contact each other.

Step 3

Turn the furnace on and listen for an audible "click" to indicate the unit has cycled on.

Step 4

Read the meter to see if there is at least 110 volts at the terminals. If so, the igniter is faulty and needs to be replaced.