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The thermal coupler is primarily a safety device that allows the pilot light to stay light, however, in the event the pilot light does go out, the thermal coupler prevents gas from leaking into the air. The thermal coupler shuts off the gas supply to the pilot of the pilot goes out. The heat from the pilot light keeps the valve in the thermal coupler open. If the pilot goes out, the metal cools and closes the valve. When a thermal coupler fails it tends to close the valve even when the pilot is burning and the metal warm. A thermal coupler is easy to replace and you will likely have to replace one on a regular basis.