Water Heater Pilot Light Won't Stay Lit

Is it dangerous when the pilot goes out?

On its own, no. The thermocouple is there for exactly this moment. When the flame goes out, Lennox explains, the thermocouple shuts the gas valve as a safety measure, so the main burner can't fire. That's why a dead pilot means cold water, not a house full of gas.

The danger is a gas smell, or a pilot that keeps going out because the heater isn't getting enough air. A.O. Smith warns that a heater short of combustion air can release unburned fuel or carbon monoxide. Treat any odor as an emergency.

Goes out the moment you release the knob

Lennox ties this pattern to the thermocouple. The thermocouple is a small copper rod that sits in the pilot flame. While it's hot, it makes a tiny voltage that holds the gas valve open. If it's worn, dirty, bent out of the flame or loose at the valve, the valve closes as soon as your thumb comes off the button.

Bradford White's service manual gives the checks in order:

1. Make sure the thermocouple nut at the gas valve is tight. The spec is finger tight plus ¼ turn. Don't crank it.
2. Make sure the thermocouple is fully seated in the pilot bracket.
3. Make sure the pilot flame fully engulfs the tip of the thermocouple. A weak or lazy flame that only licks the side won't heat it enough.
4. Test the thermocouple with a millivolt meter. With the pilot held lit for three minutes, Bradford White says 20 to 30 millivolts means good output.

If the thermocouple tests good, the problem moves to the gas control valve. Bradford White checks the magnet and the ECO (the energy cut-off, a high-temperature safety) next, and if either fails, the whole combination valve gets replaced. Step-by-step testing is on how to test a water heater thermocouple, and the swap is on how to replace a water heater thermocouple.

One practical tip: hold the knob down longer than feels necessary. Lennox's relighting steps call for about 60 seconds. A thermocouple that's borderline will sometimes hold after a full minute and fail at twenty seconds. That's a sign it's on its way out, not a fix.

Keeps going out at random

A pilot that lights, holds, and then goes out hours or days later is a different problem. Look at these.

Drafts. An open door, a window or the air from an HVAC register can blow out a small flame. A.O. Smith lists drafts and not enough air around the heater among the things to check. Heaters in garages get this a lot.

Dirty air openings. Bradford White's manual says the pilot's primary air openings must be free of dirt and lint, and the jacket slot openings around the base need to be clear. In a laundry room, dryer lint can find its way to the burner. Vacuum around the base with the gas off.

Gas pressure swings. A.O. Smith notes that pressure changes can briefly snuff the pilot, and if it keeps happening, the cause is often in the gas lines outside the heater. Low gas pressure starves the pilot too. If other gas appliances act up at the same time, call the gas utility.

A clogged pilot tube. Dust or mineral buildup cuts gas flow to the pilot. The flame looks small and weak, sometimes yellow. Lennox says a steady blue flame is healthy and a yellow or orange one points to poor combustion.

Venting problems. Bradford White's troubleshooting also covers a blocked or badly sized vent. If the flame flickers or rolls out toward you when the burner fires, stop and call a plumber.

What A.O. Smith's own manual says to do

It's worth knowing that A.O. Smith's use and care guide doesn't ask you to diagnose anything here. Its instruction for a pilot that won't stay lit is to turn the gas control knob to OFF and call a service technician or the gas supplier. If the knob doesn't pop back up after you release it, the manual says to stop and call immediately.

Electronic-ignition models add one wrinkle. After several failed ignition attempts they can go into lockout to stop gas building up. Repeated relighting won't get past that, and it isn't meant to.

What the repair costs

HomeGuide's 2026 cost guide puts a thermocouple replacement at $100 to $325 with a plumber, or $10 to $30 for the part if you do it yourself. A new gas control valve runs $150 to $550. Most other causes on this page, like a draft or lint, cost nothing to fix once you find them.