You head to the basement, the water is cold, and the burner is dead. You relight the pilot, it catches, you hold the knob for a minute, let go, and it goes right back out. Frustrating, but I have good news: this specific failure, a pilot that lights but will not stay lit, almost always points to one cheap part called the thermocouple. It is one of the most satisfying DIY water-heater repairs there is. If your water heater is cold for some other reason, work through no hot water troubleshooting first to confirm the pilot is really the problem.
First, Relight It the Right Way
Before you condemn any part, relight the pilot correctly, because a bad relight fools a lot of people. Turn the gas control to OFF and wait five full minutes to let any gas clear. Then set it to PILOT, hold the knob (or press the igniter) down, and keep holding for a solid 30 to 60 seconds after the flame catches. That hold time is what lets the thermocouple heat up enough to keep the valve open. If it stays lit when you release, great, you were just not holding long enough. If it dies within seconds every time even with a long hold, the thermocouple is failing.
Why the Pilot Will Not Stay Lit
Here is how I triage it on a service call, most likely cause first:
| What you see | Likely cause | Fix |
|---|---|---|
| Pilot dies seconds after releasing the knob | Worn or bent thermocouple | Replace the thermocouple (below) |
| Pilot flame is weak, yellow, or flickering | Dirty pilot orifice or tube | Clean the orifice; then retest |
| Pilot blows out in gusts | Draft or backdraft at the burner | Check venting and combustion air |
| Pilot lights, thermocouple new, still dies | Failing gas control valve | Replace the gas valve (often a pro job) |
The plumber tell is the flame itself: a healthy pilot is a steady blue flame whose tip wraps the top quarter to third of the thermocouple. If the flame is small, yellow, or barely touching the tip, clean the orifice before you replace anything. A universal thermocouple kit costs a few dollars and fits most atmospheric heaters.
How to Replace the Thermocouple
- Shut the gas off. Turn the gas control valve to OFF and close the gas shutoff on the supply line. Give it five minutes.
- Disconnect the three nuts. At the gas valve you will see three lines: the thermocouple, the pilot tube, and the main burner tube. Loosen all three with a wrench.
- Pull the burner assembly. Slide the whole burner out the access door as one piece. Note how it sits, you will put it back the same way.
- Swap the thermocouple. Unclip the old one from the bracket and clip the new one in the exact same spot so its tip sits in the pilot flame. Route it back to the valve.
- Reconnect and hand-tighten plus a quarter turn. The thermocouple nut only needs to be snug, not gorilla-tight, or you crush the connection.
- Leak-check, then relight. Turn the gas back on and brush soapy water on every joint, watch for bubbles. No bubbles, then relight the pilot using the long-hold method above.
When It Is Not the Thermocouple
If you replace the thermocouple and the pilot still will not hold, suspect the gas control valve, that is a bigger part and usually a pro replacement. And one important caveat: a lot of newer heaters use electronic ignition or sealed-combustion (FVIR) designs with no standing pilot, just an igniter and a thermopile. Those do not have a serviceable thermocouple in the same way, and a failed igniter or control board on those is a job for a licensed plumber or the manufacturer. When the unit is old enough that you are sinking money into it, price a swap in cost to replace a water heater and our best water heater picks.
When to Call a Pro
Replace a thermocouple on a standard gas heater yourself, it is a fair first gas repair. But call a licensed plumber if you smell gas at any point, if you have a sealed-combustion or electronic-ignition unit, if the gas valve itself is the failure, or if the heater is leaking water from the tank, that one is terminal and means a full replacement. Deciding between gas and electric for the replacement? See gas vs electric water heaters.
Frequently Asked Questions
Why will not my water heater pilot light stay lit?
Nine times out of ten it is a failing thermocouple, the small copper sensor whose tip sits in the pilot flame. When it heats up it tells the gas valve the pilot is burning. If it is worn, dirty, or bent out of the flame, it cools, the valve assumes the pilot is out, and it shuts the gas off, so the pilot dies seconds after you let go of the knob. Drafts, a dirty pilot orifice, or a bad gas valve can do the same thing, but the thermocouple is the usual culprit.
How much does it cost to replace a thermocouple?
The part itself is cheap, usually 10 to 25 dollars for a universal thermocouple. If you do it yourself the whole repair is under 30 dollars. A plumber or appliance tech will typically charge 150 to 300 dollars for the same job once you add the service call and labor, which is why this is one of the better DIY water-heater repairs if you are comfortable working around gas.
Can I replace a water heater thermocouple myself?
Yes, on a standard atmospheric gas water heater it is a beginner-friendly job: shut off the gas, disconnect three nuts at the gas valve, pull the burner assembly, swap the thermocouple, and reassemble. The one hard rule is gas safety. Shut the gas off, work in ventilation, and leak-check every joint with soapy water before you relight. If you have a sealed-combustion or electronic-ignition unit, or you smell gas at any point, stop and call a pro.
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