How To Figure Out If The Thermocouple On Your Gas Furnace Has Gone Bad And How To Replace It

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Heating systems get quite a workout in the middle of winter when temperatures drop to their lowest degrees seen throughout Canada during the year. This is also the time when a heating system will break down and stop working – leaving you to face the frigid air undercovers to stay warm. One of the parts you should check is the thermocouple. The thermocouple operates a valve on the main gas line to the burners in the furnace. If the thermocouple fails, it will shut off the gas line and the furnace will shut down. Here is how to figure out if it's your thermocouple that's gone bad and how you can fix it.

Test Pilot Light

The first think you want to check is if the pilot light is on and working well. The heat from the pilot light is used to activate the thermocouple that sits just next to or above the pilot light. The flame on the pilot light should be bright blue with a slight yellow hue at the tip and it should reach the tip of the thermocouple. The pilot light and thermocouple are next to each other and attached to the same bracket.The color of the flame means there is enough heat being applied to the tip on the thermocouple to open the valve on the main gas line (it also shows you have gas going to the furnace).

Test Thermocouple

You will need a volt- or multimeter to test the thermocouple. The voltmeter will come with two wires with clips on the end of them. One clip is red and the other one is black.

A copper wire is used to send an electronic signal to the gas valve to tell it to open up when the tip on the thermocouple gets hot. Unscrew and remove the end of the wire going into the gas valve. Set the voltmeter to the DC mode. Place the red prong along the copper wire and put the black prong on the tip of the wire. You should get a reading of at least 26 millivolts on your voltmeter when the pilot light is applying heat to the thermocouple. If the reading is below 26 millivolts, you should replace the thermocouple.

Replacing the Thermocouple

Unscrew the bolt holding the thermocouple to the bracket and slide it out. Take the thermocouple with you to the hardware or furnace supply store to match it up against the other thermocouples to make sure you get an exact replacement part for your furnace.

Screw the new thermocouple onto the bracket and screw the wire on the other end into the main gas valve. The gas valve should now open when the tip on the thermocouple gets hot.