How To Test, Remove And Replace A Thermocouple In A Furnace

by Claude Walters

The middle of summer might not seem like a time to think about your furnace, but summer will soon give way to the colder air of autumn and winter when a working furnace is needed to keep warm. You should fire up your furnace for a few minutes while it is warm outside to make sure everything is in good working order. If the pilot light isn't staying on and the burners don't light when testing the furnace, you could have a bad thermocouple. The thermocouple is the device that opens the gas line to the burners so the furnace works. Here is how you can test, remove and replace a thermocouple.

Safety First

Make sure you turn off the electrical power and the gas to the furnace before you start removing the thermocouple.

Locating the Thermocouple

  1. Take the panel off of the furnace to expose the pilot light.
  2. The thermocouple is next to the pilot light (the one end of the thermocouple extends over the top of the pilot light so it can activated by the heat coming from the flame on the pilot light).

Testing the Thermocouple

Here is how you should test the thermocouple.

  • Get a multimeter
  • Select the resistance option on your multimeter.
  • Attach the red prong to the side of the thermocouple.
  • Attach the black prong to the screw cap that goes into the gas line.
  • Turn the multimeter on.
  • If the monitor screen shows an "OL" on it, then the thermocouple is bad and you'll have to remove and replace it with a new one.

Removing the Thermocouple

Remove the thermocouple by following these procedures:

  1. Disconnect the electrical wire from the thermocouple and the gas line.
  2. Remove the nut under the bracket holding the thermocouple in position.
  3. Pull the thermocouple out of the furnace.

Replacing the Thermocouple

You should take the old thermocouple with you to the hardware store or furnace service center. You want to make sure you get an exact replacement part for your furnace so you don't run into trouble with the installation and operation of the thermocouple.

Once you get the proper part, you can reinstall it into your furnace by sliding the thermocouple back into the bracket and tightening the screw (make sure the end of the thermocouple is positioned over the pilot light so the flame can heat up the end).

Reconnect the electrical wire and the gas line.

Final Test

Restore the electrical power and open the main gas line going to the furnace. Turn the furnace on and wait a few minutes. If the burners light up and hot air starts to circulate throughout your home, you are done. Put the cover back on the furnace and go back outside to enjoy your summer day.