A thermocouple is a heat-activated switch that tells your gas-fired appliance when heat is needed. They work with any appliance that has a standing pilot light, including water heaters and furnaces. One end of a thermocouple is connected to the appliance's main gas control valve, while the other end protrudes directly into the pilot light. To disable thermocouples, either one end or the other needs to be removed from the source.
Things You'll Need
Turn off the gas on the main line that supplies your gas fired appliance. The gas valve handle must be rotated so that it lies perpendicular to the pipe.
Remove the thermocouple access panel. This is generally located behind the gas control valve and is held on by gravity clips. Grasp the sides and pull the panel up and away.
Locate the thermocouple proper. There will be a small tube that runs off the side of the gas control valve and into the area of the main burner. A flashlight will illuminate the area and make it easy to see.
Disconnect the nut that attaches the thermocouple to the gas control valve with your wrench. Realistically, you have just disabled the thermocouple, but you can also reach in and unclip it from its holding bracket where it meets the pilot light. In most cases it will be held on with a gravity clip, but some are held on with a screw that must be removed with a small screwdriver.
Pull the thermocouple out fully and replace the access door.
Use care when dealing with gas lines. Ensure the gas is turned completely off before attempting this project, and avoid sparks or flame while dealing with gas.