Why won't my water heater pilot stay lit?

Updated August 2026

On a gas water heater, a pilot that lights and then dies the moment you let go of the control knob is almost always a dirty or failing thermocouple, the sensor that proves the pilot flame and holds the gas valve open. When it stops sensing heat, the valve shuts the gas off as a safety measure. Cleaning or replacing the thermocouple, a $10 to $20 part, fixes the vast majority of these. A clogged flame-arrestor screen or a draft are the next things to check.

Safety first: this is a gas appliance, so treat a persistent problem with respect. If you smell gas at any point, near the floor especially since natural gas is odorized and can pool low, stop, leave the house, and call your gas utility from outside; do not keep clicking the igniter. Follow the exact lighting sequence printed on the tank, wait the full time it specifies for gas to clear before relighting, and never force the control knob or bypass the safety. If relighting fails two or three times, stop and call a pro rather than flooding the compartment with gas.

Causes, most likely first

What makes a furnace do this, most to least common (Updated August 2026)
Cause	How likely	The fix	Typical cost
Dirty or failed thermocouple	Most common	Clean the tip or replace the thermocouple; it must sit right in the pilot flame to keep the valve open.	$10–20 part
Clogged flame-arrestor screen at the base	Common	Lint and dust choke the air-intake screen under the tank. Vacuum it clean so the pilot gets air.	Free
Draft blowing the pilot out	Occasional	Wind through a nearby door, a strong downdraft, or a backdrafting flue snuffs the flame. Fix the draft source.	$0–200
Clogged pilot orifice or tube	Occasional	A weak, floppy pilot flame means the orifice is partly blocked. Clean or replace it for a crisp blue flame.	$0–150
Failing gas control valve	Rare	A worn valve will not hold even with a good thermocouple. A pro tests and replaces the gas valve.	$150–300

Check these first

Watch what happens when you release the knob. If the pilot dies within seconds of letting go, the thermocouple is not proving the flame, which is the classic sign.
Look at the pilot flame color and shape. A strong, steady blue flame that wraps the thermocouple tip is right; a weak yellow or lifting flame points at a dirty orifice or a starved intake.
Vacuum the flame-arrestor screen at the base of the tank, where lint and pet hair collect and choke the air supply.
Check for drafts. A door, a nearby dryer, or an open window that lets wind reach the burner compartment can blow the pilot out.
Follow the lighting instructions exactly, holding the knob the full 30 to 60 seconds the label specifies before releasing.

Prevent it next time

Pilot trouble is usually a cleanliness problem. Vacuum the flame-arrestor screen and the burner compartment a couple of times a year so the pilot always gets clean air, and keep laundry lint, dust, and stored boxes away from the base of the heater. Fold pilot and burner care into your yearly routine alongside a tank flush, an anode-rod check every 3–5 years, and a TPR-valve test. A clean, well-kept gas heater holds its pilot reliably across the typical 8–12 year tank life.

When to call a pro

Call a licensed plumber or gas tech when a new thermocouple and a clean intake screen still will not hold the pilot, when the flame stays weak and yellow after cleaning the orifice, or when you suspect the gas control valve. A thermocouple replacement runs $150–300 installed, a gas-valve swap $200–400, and a full diagnostic $100–200. Stop and call immediately if you smell gas or see soot around the burner; a gas appliance is not the place to keep guessing past two failed relights.

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Frequently asked questions

How do I know if it is the thermocouple or the gas valve?

Timing and flame quality tell you. If the pilot lights and holds while you press the knob but dies the second you release it, the thermocouple is almost certainly the problem, because its job is to keep the valve open after you let go. If the pilot itself is weak or will not light despite a clean orifice and good gas, or if a brand-new thermocouple still will not hold, the gas control valve is the likelier culprit and a pro should test it. Start with the cheap $10 to $20 thermocouple.

Is it safe to relight a water heater pilot myself?

Yes, when you follow the label and there is no gas smell. Read the lighting sequence on the tank, turn the knob to pilot, hold it while you light, and keep holding for the full time it specifies. If you smell gas before or during, stop, ventilate, and wait; do not spark an igniter into a gas-filled compartment. If it fails two or three careful attempts, leave it off and call a pro. Relighting is routine, but a repeatedly failing relight is a signal, not a task to force.

Why does the pilot stay lit for a while, then go out hours later?

A pilot that holds and then quits later usually points at a marginal thermocouple or an intermittent draft rather than a total failure. A thermocouple weakening with age can make just enough signal to hold at first, then drop below the threshold as it heats and cools. A backdrafting flue or wind that only kicks up sometimes will snuff it unpredictably too. Replace the aging thermocouple first, then look for draft sources at the flue and burner compartment if it still cuts out.