Thermocouple

A thermocouple is a pair of dissimilar metals joined together that produce a measurable electrical voltage when exposed to temperature change. In plumbing and HVAC work, it's the safety device that tells a gas valve the pilot light is actually burning. When the pilot flame heats the thermocouple junction to roughly 700°F, it generates about 25–30 millivolts of DC current, which keeps the gas solenoid valve open and allows the main burner to fire. If the pilot goes out, the thermocouple cools instantly and voltage drops to zero, triggering the solenoid to shut and cutting gas flow.

Thermocouples are critical on tank-style water heaters, furnaces, boilers, and standing-pilot stoves. They're cheap, reliable, and require no electricity to function—just heat. The junction sits directly in the pilot flame and needs clear, consistent contact with the heat source. A weak or dirty thermocouple won't generate enough voltage to keep the solenoid cracked, so the pilot won't stay lit even if you get it lit manually.

When troubleshooting a pilot that won't hold, the thermocouple is the first thing to test. Use a multimeter set to DC millivolts across the two terminals with the pilot running; you should see 20–30mV. Below 15mV usually means replacement. Soot buildup on the junction, a loose connection at the solenoid, or a kinked tube can all drop your reading and kill the pilot. Replacement is straightforward and inexpensive.