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Water heater pilot light won't stay lit: causes and fixes

On a standing-pilot gas water heater, the small pilot flame lights the main burner. If it goes out, you lose hot water. When it lights but goes out as soon as you let go of the knob, the cause is usually one of a few parts, and most are inexpensive.

Safety first. If you smell gas (a rotten-egg odor), do not try to light anything, don't flip switches, and leave the house. Call your gas utility or 911 from outside. Never relight a heater that has been under water from a flood; the gas control needs to be replaced by a professional.

First, know what kind of heater you have

Standing pilot: a gas control knob with "OFF / PILOT / ON" and a pilot you can see through a small window. This guide is mainly about these.
Electronic ignition / hot surface igniter: no constant pilot; the heater lights itself when it calls for heat. These have different parts (igniter, flame sensor, control board) and usually show error codes or blinking lights. Check the manual for what the code means.
Sealed combustion with a flammability sensor (FVIR): most gas tank heaters made since the mid-2000s have a sealed burner chamber and a sensor that shuts the heater down if it detects flammable vapors or overheating. You light the pilot with a built-in push-button igniter, not a match.

How to relight the pilot

Follow the lighting instructions printed on the label on your heater; they're specific to that model. The general pattern is:

Turn the gas control knob to OFF and wait the time the label says (often around 5 to 10 minutes) for any gas to clear.
Turn the knob to PILOT, then press and hold it (or the separate pilot button) to send gas to the pilot.
Click the igniter button until the pilot lights, while still holding the knob down.
Keep holding for the time the label says, commonly 30 to 60 seconds, so the thermocouple heats up.
Release slowly. If the pilot stays lit, turn the knob to ON and set the temperature.

If the pilot goes out the moment you release the knob, keep reading.

Common causes and what to check

1. Thermocouple (or thermopile) is bad, dirty or out of position

The thermocouple is a thin copper-colored rod that sits in the pilot flame. When hot, it makes a small voltage that holds the gas valve open. If it's worn out, coated with soot, or bent away from the flame, the valve closes as soon as you let go. This is the most common cause of a pilot that won't stay lit.

Make sure the tip sits in the flame, with the flame wrapping around the top portion of it.
Check that its nut at the gas control valve is snug (hand tight plus a small turn; don't overtighten).
Thermocouples are inexpensive and replaceable on many older open-burner heaters. On sealed FVIR heaters, the thermocouple is often part of a pilot assembly that's replaced as a unit, and getting the burner door gasket resealed correctly matters.

2. Weak, yellow or flickering pilot flame

A healthy pilot flame is mostly blue and steady and covers the thermocouple tip. A small, yellow or lazy flame often means a dirty or partially clogged pilot orifice or tube, so not enough heat reaches the thermocouple. Cleaning the pilot assembly is doable but fiddly; on sealed burners it's often a job for a pro.

3. Flammable vapor or thermal cutoff tripped (FVIR heaters)

If the sensor detects vapors (gasoline, paint thinner, solvents stored nearby) or overheating from a blocked air intake, it shuts the heater down. On many models it can't be reset; the sensor or pilot assembly has to be replaced. Check the manual or the status light, remove any stored chemicals, and clear the air intake screen at the bottom of the heater, which can clog with lint, dust and pet hair.

4. Drafts or venting problems

A strong draft can blow the pilot out on older open-burner heaters. A blocked, disconnected or poorly drafting vent can starve the flame of air or cause the heater to shut itself down. Look at the vent pipe on top of the heater to make sure it's connected and not rusted through or blocked. If you suspect a venting problem, have it checked; poor venting can put carbon monoxide into the house. Make sure you have working CO alarms.

5. Gas supply issues

Confirm the gas shutoff valve on the line to the heater is open (handle parallel to the pipe).
Check whether other gas appliances work. If none do, the problem may be the supply or the meter; call your utility.
On propane, check the tank level. A low or empty tank, or air in the line after a refill, can keep the pilot from staying lit.

6. Bad gas control valve

If the thermocouple is new and in the flame, the pilot flame is good, and it still won't stay lit, the gas control valve (thermostat) may have failed. This part controls the gas flow, so replacement is best left to a licensed plumber or gas technician. On an older tank, a failed control valve is often the point where people weigh replacing the whole heater.

When to call a plumber

You smell gas at any point.
The pilot won't stay lit after you've checked the thermocouple position and flame.
The heater has an FVIR sensor that tripped, or a lockout you can't clear.
You see scorch marks, soot around the burner door, or signs of flame rollout.
The vent pipe is damaged or disconnected, or your CO alarm has gone off.

Tip: Before you call, write down the brand, model and serial number from the rating label and take a photo of the pilot flame through the window. It helps the plumber bring the right part on the first visit.