Test a gas valve thermocouple by attempting to light the pilot, holding the pilot button and releasing it to see if the light goes out, which normally indicates a nonfunctioning thermocouple. Homeowners may also remove the thermocouple to perform a closed circuit test using a multimeter and a thermocouple adaptor.
Hold the pilot button for 30 to 60 seconds before releasing it to see if the pilot light goes out. If the pilot does not light at all, the issue is likely related to the gas flow or gas valve rather than the thermocouple. A multimeter may also be used to perform several tests on the thermocouple, including a resistance test, a closed circuit test and an open circuit test.
For absolute confirmation that the thermocouple is defective, follow safety protocols while removing the thermocouple from the gas valve, and perform a closed circuit test. Place a thermocouple on the gas valve where the thermocouple normally sits, and screw the thermocouple into the adaptor. Attach one crocodile clip to the screw that sticks out of the adaptor's side and the other to the side of the thermocouple, ensuring that the adaptor is on. A normal reading is between 12 and 15 millivolts, and the thermocouple is defective if the reading falls below this range.