How to Replace a Water Heater Thermocouple

Is it easy to change a thermocouple?

On a traditional heater with an open burner door, yes. Two or three nuts, one clip, and a part that costs less than lunch.

On a sealed-combustion heater it's harder. Those burners sit behind a gasketed inner door, and on many models the thermocouple or thermopile comes as part of a pilot assembly from the heater maker. A.O. Smith's ProMax, for example, uses a thermopile with wire leads rather than a copper thermocouple. If the inner door has a gasket and screws you'd have to break, that's the point to call a pro.

Parts and tools

• A replacement thermocouple at least as long as the old one. Replacements come in several lead lengths; Resideo's Q390A series for standing-pilot systems runs from 18 to 48 inches. If your heater uses a maker-specific pilot assembly, get that part by the model number on the rating plate.
• An adjustable or open-end wrench sized for the fittings on the gas control.
• A screwdriver for the access covers and burner door.
• A flashlight.
• Soapy water in a spray bottle for the leak test.

Angi's list for this job is about the same: flashlight, adjustable wrench, screwdriver and spray bottle, plus the thermocouple and dish soap.

Step by step

Put both covers back once the pilot is stable. Then turn the knob to your temperature setting and listen for the burner to light.

Brand notes

Honeywell and Resideo gas controls. These are common on older heaters from several brands. Resideo's own Q390A thermocouples are made for standing-pilot gas systems; match the connector to your control before you buy.

American, A.O. Smith, Rheem and other tank makers. Older open-burner models usually take a universal thermocouple. Newer sealed-combustion models usually take a maker-specific pilot assembly or thermopile. Look up the model number on the rating plate before you buy.

Mistakes that cause leaks or callbacks

• Not leak-testing the gas fittings. Angi is blunt about it: do the leak test every time.
• Overtightening the thermocouple nut. Snug plus a quarter turn.
• Buying a lead that's too short, then stretching it.
• Leaving the tip off-center in the pilot flame, so the new part reads low.
• Bending the pilot tube while sliding the burner out, which starves the pilot.
• Running the heater with the burner door off.

If the pilot still won't stay lit

Then the thermocouple wasn't the problem, or wasn't the only one. Check the pilot flame (a weak, yellow pilot won't heat the tip), drafts, and, on heaters that have one, the air filter at the base, which A.O. Smith's ProMax manual says can let the pilot go out if it isn't cleaned. If those are fine, the magnet in the gas control may be failing. A.O. Smith says a failed pilot control valve usually means a service visit and likely a new valve. See water heater gas control valve problems.

When to hand it to a pro

Call a plumber if the heater has a sealed burner chamber you'd have to unseal, if any gas fitting won't pass the bubble test, or if you'd rather not do gas work. That's a fair call. HomeGuide puts thermocouple replacement at $100 to $325 with a pro, and service call fees of $50 to $200 may apply.