The thermocouple is a vital safety feature for many gas appliances. A thermocouple uses the heat from a flame to produce a millivoltage that controls a gas valve. If the pilot goes out, the thermocouple can no longer send the millivoltage that is required to allow the gas valve to stay open, thus sealing the valve to prevent a dangerous gas leak.
Remove the thermocouple from the appliance. Make sure all safety protocols are followed. Make sure that the multimeter to be used can read both ohms/resistance and millivolts.
Choose a test to assess the thermocouple. The three main tests used are the open circuit test, closed circuit test and resistance test. While all three are useful, the closed circuit test will clearly indicate whether the thermocouple performs properly under load and is often the primary test conducted.
Select the ohms or resistance option on your multimeter. Place one test lead on the side of the thermocouple and the other to the end that screws into the gas valve. If the thermocouple has proper continuity, a small resistance reading should be visible on the multimeter. Many multimeters also indicate continuity with an audio alert. If the reading on the multimeter indicates an "OL" reading, the thermocouple does not have continuity and is therefore not usable. A high resistance reading, such as 40 ohms, suggests that the resistance will cause problems and the thermocouple should be discarded.
Turn your multimeter to millivolts to perform an open circuit test. Place one of the test leads of the multimeter on the side of the thermocouple and the other to the end that enters the gas valve. Ignite a lighter, or other heat source with a flame, at the opposite end of the thermocouple. Thermocouples designed for residential gas appliances, such as fireplaces, water heaters and furnaces, are made for 30 Millivolts. The reading should be between 25 and 30. Anything under or hovering around the 20 millivolts mark means the thermocouple needs to be replaced. Discretion should be used if the reading is between 21 and 25.
Perform a closed circuit test using the thermocouple adaptor to achieve a true performance ability under load. It is easier to perform this test using crocodile clips attached to the test leads. First, attach the thermocouple adaptor to the gas valve in the thermocouple's spot. Then attach the thermocouple as normal with the exception that the end that would screw into the gas valve will screw into the adaptor.
Turn the appliance on and place a crocodile clip from the multimeter to the side of the thermocouple. Attach the other crocodile clip on the screw that extends out of the side of the adaptor. The reading should be within 12 millivolts and 15 millivolts. Fewer than 12 millivolts signifies that the thermocouple is defective.
Make sure that the multimeter can read both ohms/resistance and millivolts.