How to Replace a Furnace Thermocouple. As with anything mechanical, various parts of a furnace can and do wear out. One important part is the thermocouple. The thermocouple is a safety feature that shuts off the gas if the pilot light goes out or if there is an igniter failure. You must change a bad thermocouple as soon as possible. After you replace it, rest easy knowing your furnace is working properly and safely.
Shut off the gas valve for the furnace. Remove screws from the front panel and remove, exposing an upper and lower panel from which you can access the thermostats. Unscrew top front panel, and remove to access the thermocouple block.
Locate the thermocouple next to the pilot light. Disconnect the thermocouple by removing the copper lead, unscrewing the gas line connection nut and the bracket nut holding the thermocouple tube in place. The nut is located underneath the mounting bracket.
Replace the old thermocouple. Remove it, and slide the new thermocouple into bracket hole with the steel tube facing up and the copper tube facing down. Tighten bracket nut underneath bracket to secure tube.
Connect the copper lead to the gas line. Tighten the connection nut. Make sure the connection is clean and dry. Hand-tighten all the nuts. To replace the lower thermostat, remove lower access panel and repeat steps.
Turn on the gas supply to the furnace. Relight the pilot light by following the furnace manufacturer's instructions. Make sure the pilot light stays lit to ensure the new thermocouple is working properly.