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How To Replace a Water Heater Thermocouple

All gas-fired water heaters use a thermocouple (or thermopile in some advanced models) in the pilot/main burner assembly. The purpose of the thermocouple is to keep the pilot side of the main gas valve open allowing a full gas supply to flow to the pilot burner. The thermocouple senses the pilot flame and holds the pilot side of the main gas valve open. If the pilot flame should go out, the thermocouple will shut off the gas supply to the pilot and not allow the main gas valve to open. In that respect, it is a safety device. If the pilot does not light or lights and immediately goes out, suspect the thermocouple. Replacement of the thermocouple usually means disassembling the burner assembly.

Preparation

Pilot won't stay lit? You probably need to replace the thermostat

Turn the water heater thermostat to the off position.
Close off the gas shut off valve to the appliance.
Remove the screws holding the burner compartment door in place and remove the door.
Lift up and remove the sliding burner compartment door inside the water heater jacket.

Removing the burner assembly

Using the proper size open end wrench, loosen the small compression nut on the pilot burner tube on the bottom of the main gas valve.
In the same way, loosen the thermocouple nut, also found on the bottom of the gas valve in most cases.
Using a larger open end wrench, loosen and remove the main burner tube nut that holds the main burner tube in the bottom of the main gas valve. The main burner tube nut is usually a left hand thread nut.
The entire burner assembly is now loose. You can pull the burner assembly out slightly, twist it so the burner clears the burner compartment door, and remove the burner assembly from the water heater.

Replacing the thermocouple

Locate the two small screws that hold the pilot assembly in place on the main burner.
Remove both small screws and pull the pilot assembly away from the burner assembly.
The thermocouple is usually held in place in the pilot assembly with a clip or sleeve that pushes into place. Remove it and pull the thermocouple free of the assembly.

Installing the new thermocouple

Uncoil the new thermocouple and install the thermocouple end in the pilot assembly.
Reinstall the pilot assembly in the main burner assembly. Make sure the pilot assembly is pointing properly at the main burner and that the thermocouple length is adjusted so that the thermocouple tip is immersed in the full pilot flame.
Reinstall the main burner assembly in the burner compartment. Verify that the tip of the main burner assembly engages the burner support tab inside the burner compartment.
Gently reinstall the main burner tube in its port in the main gas valve and start the nut.
Reinstall the pilot burner tube in its port.
Route the thermocouple tube so it is not kinked or twisted and insert the end into the main gas valve thermocouple port. Start the thermocouple nut.
Tighten all three nuts firmly but gently. It is easy to strip out the threads in the aluminum main gas valve while trying to start the brass nuts.
Make sure that all three lines are firmly attached to the gas valve.
Turn on the appliance gas shut off valve.
Follow the manufacturer’s instructions to light the pilot.
Replace the interior sliding burner compartment door.
Replace the exterior burner compartment door.
After verifying that the pilot is burning properly, turn the water heater thermostat to the desired temperature. The main burner should ignite and burn properly.
Using a soap and water solution, check all the connections for tightness and gas leaks. If bubbles appear at any connection, no matter how small the bubbles may be, there is a leak at that point. Turn off the gas and retighten that connection. Recheck it for a leak with the leak test solution.