How to Replace a Thermocouple on a Water Heater in Under an Hour

Before beginning any work on any gas water heater, you'll need to turn off the gas supply. If you have a thermocouple, you'll also have a standing pilot light, which means gas is attempting to flow to the unit at all times.

Locate the temperature control valve on the heater and turn it to its lowest setting. Then find the gas valve on the gas supply line entering the valve. It's not far away from the heater and resembles a lever or a knob. Turn the valve fully clockwise to turn off the flow.

Find where the small gas line and thermocouple connect to the unit at the bottom or side of the heater control valve. The gas line is less than a quarter-inch in diameter, and the thermocouple connector looks like a large-gauge copper wire. Use an adjustable wrench to turn the connection nuts counterclockwise until they are free of the control valve box. 

Shine a flashlight into the burner chamber to identify any clips or nuts holding the burner assembly in place. Use a screwdriver or adjustable wrench to disconnect the component if necessary. In many cases, no attachment points will exist. Remove the entire assembly from the water heater.

Note the position of the thermocouple in the burner assembly and consider taking a photo for later reference. Use a screwdriver or adjustable wrench to loosen the screw or nut holding the thermocouple in place. You may also be able to simply pull the component out with your fingers.

Take the thermocouple to your local hardware store or home center to find the appropriate replacement part. Take particular care to purchase one that's the same length as the old one. Install the new thermocouple in the burner assembly precisely as the old one was. Tighten any attachment screws or nuts. Uncoil the new thermocouple wire before the next step. 

Reinsert the burner assembly into the burning chamber and reattach any screws or clips to secure the component. Use an adjustable wrench to reinstall the thermocouple connector and the gas line into the appropriate holes in the control valve.

Turn the gas back on to your water heater and reignite the pilot light. Once lit, mix a few drops of dish soap with a few ounces of water in a spray bottle or something similar. Spray the gas connection at the control valve until it's thoroughly wet but not dripping wet.

Watch for bubbles to form near the connection. If bubbles are present, you have a gas leak. Turn off the gas and tighten the connection. Test it again. If the leak persists, turn off the gas again and call a water heater professional near you for help. If no bubbles are present, wipe off the soap mix and adjust the water heater to your desired heat setting.