Testing an Ignition Module on a Furnace


A gas furnace that will not light consistently, or at all, may have a bad ignition module inside. Modern gas furnaces use an igniter that glows orange when sufficiently hot to light the gas. The gas valve opens only when the igniter reaches the preset temperature. The igniter is part of the ignition module that plugs into an electrical socket inside the furnace. Testing the ignition module takes only a few minutes using an ohmmeter.

Things You'll Need

  • Nut driver
  • Ohmmeter
  • Screwdriver
  • Shut down the furnace at the thermostat control, then disconnect the electricity at the circuit breaker. Wait for the furnace to cool before testing the ignition module.

  • Extract the bolts in the service cover plate on the side of the furnace. Remove the plate and set it out of your way. The ignition module is behind the cover plate, near the bottom and next to the gas valve. The module consists of a flat igniter tip made of thin metal wired to an electrical plug. The igniter tip mounts in position next to the gas valve on a metal bracket.

  • Unplug the power plug on the wired end of the igniter from the electrical socket inside the furnace. Pull the plug itself, not the two wires.

  • Dial the ohmmeter to "Rx1" and touch the two probes on the wired ends to each other. A beeping sound means the ohmmeter is operating and ready for testing.

  • Place each ohmmeter probe on one of the metal contacts in the power plug on the end of the module. If the meter displays zero with the furnace at room temperature, the ignition module is faulty and must be replaced.

  • Remove the screw holding the ignition module to the metal bracket inside the furnace. Take the defective module with you to an HVAC (heating, ventilation and air conditioning) repair shop to match with the correct replacement.

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