Exploring Emergency Plumbing Services

How To Check A Gas Furnace Thermocouple

by April Rivera

If your gas fireplace won't light or it lights then goes out, test the thermocouple. The thermocouple is a heat-sensing device that controls the furnace to prevent gas leaks. You should be sure it is the thermocouple causing problems before you replace it. Here are tips to test a gas furnace thermoocouple.

Prepare to Work

For this project, you need:

  • work gloves
  • cloths
  • compressed air
  • screwdriver, sandpaper, or steel wool
  • long match (optional)
  • mutlimeter with alligator clips

Shut off gas to the fireplace, which is commonly located on the gas line attached to the burner. Rotate the valve 90 degrees to the right, and then let the unit cool.

Locate the Pilot and Thermocouple

Look for the pilot along the movable grill on the bottom of the fireplace. Detach the cover plate screws with a screwdriver. The pilot is commonly a blue L-shaped tube.

The thermocouple consists of two metal parts connected with a wire. The welded side is the hot junction, and the non-welded side is the cold junction. It should be directly over the pilot.

Test the Pilot

Clean soot from the exterior of the thermocouple and pilot with a rag, if needed. Spray compressed air inside the pilot tube.

Check your manual for directions on how to light the pilot. A common procedure is to set the gas valve on "pilot," press and rotate the knob, then push the red igniter button. On an older furnace, you may need to use a match. Keep the control valve pressed for about one to two minutes, then release. If the pilot burns a steady blue, the thermocouple isn't defective.

Sometimes, a draft could separate the pilot from the flame, even when it lights. Ensure the thermocouple stays on the top third of the blue flame while the pilot burns. The flame should make contact with the tip of the themrocouple. If the pilot fails, relight it and inspect the flame again

Test with a Multimeter

If the pilot won't stay on, test with a multimeter. Hold the thermocouple's hot junction with pliers. Attach one probe to the end of the silver end of the wire that touches the gas valve, and attach another probe to the copper wire.

Light the pilot, and press in the gas valve. Hold the valve for a minute. A good reading should be between twenty and thirty millivolts. If you get a reading lower than twenty millivolts, replace the thermocouple. Remove the wrench.

Reattach the access panel. If you don't trust your skill to replace the pilot or you can't find the problem, contact an HVAC service like Bill Rhiner's Plumbing, Heating & Cooling.