How to Test a Thermocouple on a Gas Valve

Inside your gas-operated water heater is a tiny gas flame called a pilot light, whose purpose is to ignite the burner whenever the temperature falls below the thermostat setting.

Right next to the pilot light is a small metallic tube known as a thermocouple. This is a safety device, whose purpose is to monitor the pilot light and send a signal that shuts the gas burner valve should the pilot light ever go out -- and keep it shut until the thermocouple senses that the pilot light is lit. If the thermocouple stops working, the pilot light will go out, preventing the burner from working [source Family Handyman].

Here's how to test a suspected thermocouple in a gas operated water heater:

Light the pilot by pressing and holding the pilot light button on the gas control valve. If you can't light the pilot, the problem most likely lies with the gas control valve or the gas flow itself, not the thermocouple.
Continue pressing the pilot button for 30 to 60 seconds after lighting the pilot light.
Release the button. If the pilot light goes out, the trouble is most likely a defective thermocouple [source: Lashley].