The number
Fixr's repair guide: replacing a thermocouple in a water heater should cost between $150 and $325, labour included. Fixr calls it an integral part of the pilot light assembly, and notes it can wear down over time and interfere with the functionality of the pilot light.
What it actually does
This Old House describes the control chain on a gas unit: gas water heaters use a burner at the bottom of the tank, and a series of controls regulate temperature and pressure, including thermostats, thermocouples and pressure relief valves, all working together to maintain peak water heating conditions and prevent potential hazards.
The thermocouple's job inside that chain is narrow and safety-critical. It sits in the pilot flame, and while it is hot it holds the gas supply to the pilot open. When the flame goes out, the thermocouple cools within seconds and the gas shuts off. That is why the classic symptom is so specific.
The symptom that points at it
The pilot lights while you hold the button, then goes out when you let go. That is the thermocouple failing to confirm the flame. It is not a sign the gas is off, the burner is broken or the tank has failed.
Distinguish it from the neighbouring faults:
| What you see | Likely part | Cost (Fixr) |
|---|---|---|
| Pilot is simply out, relights and stays lit | Pilot light, relight or repair | $75 to $175 |
| Pilot lights, dies as soon as you release the button | Thermocouple | $150 to $325 |
| Pilot stays lit but the burner never fires | Gas valve | $150 to $300 |
| No hot water, unit is electric | Element or thermostat | $100 to $750 |
This Old House frames the first check the same way: the pilot may not be lit, which is a relatively easy fix, while the absence of hot water could indicate a more serious issue such as a failed thermostat, heating element or gas control valve.
Why the range runs to $325
The part is cheap and standard. What varies is access and the diagnosis around it. A thermocouple on an open-standing garage unit is a short visit; one on a unit in a tight closet, behind a sealed combustion chamber cover, takes longer. And a plumber who finds a sooty burner or a backdraft problem is no longer doing a thermocouple job. Fixr prices backdraft at $100 to $500 separately.
Do not treat a repeat failure as bad luck
Our reading, marked as ours: a thermocouple that fails twice within a year is usually reporting something else. Repeated failures point to a draught problem, a partially blocked flue or an over-firing burner, all of which keep the pilot unstable. Paying $150 to $325 a second time without asking why the first one failed buys the same outcome again. Fixr publishes the prices and This Old House the control chain; neither draws this conclusion.
On a gas unit that is also backdrafting, the safety stakes are not theoretical. See water heater venting cost.
Electric units do not have one
Fixr calls the thermocouple an integral part of the pilot light assembly. An electric tank has no pilot assembly, so it has no thermocouple. That conclusion is ours, drawn from Fixr's own description of the part. If your unit is electric, the equivalent parts are the thermostat at $100 to $250 and the heating element at $200 to $750.
Sources
- Fixr, Water Heater Repair Cost, updated August 29, 2025
- This Old House, Why Water Heaters Fail, updated September 16, 2024
- This Old House, Winter Water Heater Problems, updated May 8, 2026
Price figures are national US ranges from the sources above as of the date shown. Your local price depends on labor rates, permits and site conditions. Read how we get our numbers.