How To Replace A Thermocouple On A Gas Furnace

A furnace that won't light can be the result of a bad thermocouple. The thermocouple allows gas to flow to the pilot light and then on to the burners to produce the heat to warm your home. If the pilot isn't lighting up, it can be the result of the thermocouple going bad. Here is how you can test and replace the thermocouple.

You Will Need:

  • Adjustable Wrench

Test Pilot Light First

You will want to make sure it is the thermocouple that is broken, and that the problem is not a clogged pilot light. Take a thin piece of wire and poke it through the hole on top of the pilot light to clean it out. Try lighting the pilot after you have removed any residue that might have built up inside it and around it. If the gas fails to go through the pilot light and you don't get a flame, you can go ahead and replace the thermocouple.

Think Safety First

You should shut the gas off going to the furnace. You can shut the gas off at the pilot light control button, or at the switch along the gas line going to the furnace. The switch on the gas line has an arm on it that will tell you if the gas is on or off. Turn the switch so the handle is perpendicular to the gas line – this is the off position. You also want to turn the electricity running to the furnace off at the circuit breaker to completely shut the furnace down while you work on it.

Remove Thermocouple

Loosen the thermocouple nuts with an adjustable wrench and unscrew them with your hand (make sure everything is cooled down so you don't burn yourself). You need to disconnect the thermocouple from the gas control valve before you pull it out of the furnace – you can find the disconnect clip by following the wire on the end of the thermocouple to the gas line.

Get New Thermocouple

You will want to make sure you obtain a replacement thermocouple that is meant to be specifically installed into your particular furnace. Your local furnace supply store can assist you if you have any problems getting the right part number.

Install New Thermocouple

Reconnect the wire going to the gas control valve and slide the thermocouple into the place. Replace the screws, and then turn on the gas and electricity.

Test Pilot Light

Make sure the thermocouple is positioned over the pilot light so the flame can heat it. If the pilot light stays on after you light it, you are done. If it doesn't, there may be a bigger problem. At this point, it's best to contact a heating contractor for assistance.