A thermal coupler, also called a thermocouple, consists of two strips of different metals that join at a point. When the point where the wires join is heated, the thermocouple produces electricity. In homes, these devices are used to control the gas valves in older furnaces, stoves and water heaters. The thermocouple is in the middle of the pilot light flame. If the pilot light goes out, the thermocouple stops producing electricity and a solenoid closes the gas valve to prevent gas from leaking. If your pilot light will ignite, but goes out as soon as the pilot button is released, your thermocouple may be defective.
Disconnect the thermocouple from the thermostat. Leave the other end attached to the gas valve. The end you disconnect should have a silver colour, while the other end should be copper.
Clip the black lead of a multimeter to the end of the thermocouple part you just disconnected. Clip the red lead of the multimeter to the copper part of the thermocouple. Turn on the multimeter. Set it to detect millivolts.
Hold down the pilot light button and look at the thermocouple. Its tip should still be in the centre of the flame. If it isn't, release the pilot light button, wait half a minute for the thermocouple to cool and readjust its position.
Light the pilot and hold down the button to keep it lit. Watch the multimeter for 45 seconds. If it reads 12 millivolts or more, the thermocouple is still good. If it reads lower voltage or no voltage, replace it.