Water Heater Pilot Light Won't Stay Lit: Thermocouple Fix

A gas water heater pilot light that lights but won't stay lit is almost always caused by a failed thermocouple — a $15–$25 safety device that tells the gas valve the pilot flame is present. Without a working thermocouple, the gas valve automatically shuts off the gas supply (a safety feature). Replacing the thermocouple is a straightforward repair that requires no special tools and takes about 30 minutes.

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Common Symptoms

Pilot light goes out immediately when you release the pilot button
Pilot lights successfully but main burner won't fire
Pilot flame is very small or yellow instead of blue
No hot water despite attempting to relight the pilot multiple times
Clicking or gas smell when attempting to light the pilot

Most Likely Causes

1
Failed Thermocouple (Most Common)
The thermocouple is a thin probe that sits in the pilot flame. It generates a small electrical voltage from heat, which signals the gas valve to stay open. When the thermocouple fails, it stops generating this signal and the gas valve closes — extinguishing the pilot within seconds of releasing the control button. Thermocouples typically last 10–15 years and cost $15–$25 to replace.
2
Dirty or Clogged Pilot Tube
The pilot tube is a small copper or steel tube that delivers gas to the pilot burner. Dust, debris, or a spider web can clog the tube, producing a weak or yellow pilot flame that doesn't reach the thermocouple tip properly. Cleaning the tube with compressed air often restores the pilot.
3
Failing Gas Control Valve (Thermostat)
If a new thermocouple doesn't solve the problem, the gas control valve itself may have failed. The valve contains the pilot valve, main gas valve, and temperature control in one unit. Replacement costs $100–$200 and is often close to the cost of a new water heater on an older unit.
4
Draft or Exhaust Backdraft
In some installations, air drafts from nearby HVAC vents, open windows, or negative air pressure in the home can blow out the pilot flame. A draft hood that's damaged or missing can also allow exhaust gases to be drawn back through the combustion chamber, extinguishing the pilot.

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Quick DIY Checks

Safety Warning

If you smell strong gas at any point, do not attempt to light the pilot. Leave the house immediately and call your gas utility company from outside. Never force-light a pilot if there's a persistent gas smell — gas may have accumulated and a spark could cause an explosion.

Caution

Always wait at least 5 minutes after turning the gas control knob to OFF before attempting to access internal components or relight. This allows residual gas in the burner chamber to clear.

1Attempt to relight the pilot first: turn the gas control knob to PILOT. Hold down the pilot button (or control knob) and spark the igniter repeatedly. Keep holding the button for a full 60 seconds after the flame lights — this gives the thermocouple time to heat up. Slowly release the button. If the flame holds for 60 seconds then goes out when you release the button, the thermocouple is definitely the issue.
2Inspect the pilot flame: when the pilot is lit, observe the flame. It should be a steady blue flame wrapping around the thermocouple tip. A small, yellow, or flickering flame indicates a dirty or partially blocked pilot tube — or insufficient gas pressure.
3Clean the pilot tube: turn off the gas control knob to the OFF position and wait 5 minutes for gas to clear. Use a can of compressed air or a thin wire to gently clear any debris from the pilot tube orifice. Avoid using anything that could puncture or enlarge the orifice.

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4Replace the thermocouple: turn the gas control knob to OFF and wait 15 minutes. Locate the thermocouple — it's a thin copper rod running from the pilot assembly to the gas control valve (screws into the valve). Unscrew it by hand or with a small wrench (usually 7/16 inch). Take the old thermocouple to a hardware store or order the same length online. Install the new one by threading it in snugly — do not over-tighten.
5Check for drafts: after replacing the thermocouple and relighting, observe whether the pilot stays lit over several hours. If the pilot blows out again, check for draft sources — HVAC registers, windows, or exhaust fans near the water heater. Close nearby vents temporarily to diagnose a draft issue.

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Repair vs Replace

✓ Worth Repairing

A thermocouple is one of the cheapest appliance repairs possible — $15–$25 in parts and 30 minutes of work. Even a gas control valve replacement ($100–$200) is worth doing on a water heater under 10 years old. The average gas water heater lasts 12–15 years — repair makes sense until the unit is near end of life.

Est. Repair Cost

$15–$30 for thermocouple (DIY)

Est. Replacement Cost

$800–$1,500 for a new gas water heater installed

Recommended Tools & Parts

Universal Thermocouple (24-inch)
Universal thermocouple compatible with most gas water heaters. Available in 18, 24, and 36-inch lengths — measure your existing thermocouple before ordering.
$10–$20
Buy on Amazon →
Gas Water Heater Thermocouple and Pilot Assembly
Complete pilot assembly kit including thermocouple, pilot tube, and igniter wire. Brand-specific kits available for AO Smith, Rheem, Bradford White, and more.
$20–$40
Buy on Amazon →
Gas Control Valve (Thermostat Valve)
Replacement gas control valve/thermostat for gas water heaters. Required if thermocouple replacement doesn't fix the pilot. Match BTU rating and gas type to your unit.
$100–$200
Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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