What is the risk of a thermocoupler failing??? A thermocoupler is designed to be a fail safe. It generates a small voltage if the pilot light is on, which lets the water heater know to allow gas to flow openly. Gas will only flow openly if the pilot light is on to hear the thermocouple. A failed thermocouple will always shut off the gas, so I can't imagine anything else happening to a simple design. If something serious happened to the water heater, it seems like it would not be due to a modification to the thermo couple. Please, anyone, correct me if in wrong.