Water Heater Pilot Won't Stay Lit
When a pilot lights briefly but extinguishes as soon as you release the pilot button, the thermocouple is the problem in 90% of cases. The thermocouple generates a small electrical signal (20–30mV) that tells the gas valve to stay open — when output drops below ~15mV, the gas valve shuts off as a safety measure. Thermocouple replacement is a 15-minute job that costs $12–$20 in parts and requires only basic tools.
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Common Symptoms
- Pilot lights when button is held down but goes out when button is released
- No hot water — water heater not firing at all
- Had to relight the pilot multiple times recently
- Pilot flame is very small or weak-looking
- Thermocouple tip is covered in carbon/soot deposits
Most Likely Causes
- 1
Failed Thermocouple (Most Common)
Thermocouples have a lifespan of 3–7 years under normal use. As the internal thermoelectric junction degrades, output millivolts drop below the threshold needed to hold the gas valve open. The valve shuts off pilot gas as soon as you release the button — which is exactly how the safety system is designed to work when sensing a weak or absent pilot flame.
- 2
Thermocouple Tip Not in the Pilot Flame
The thermocouple tip must be positioned directly in the pilot flame — typically 3/8 to 1/2 inch into the blue part of the flame. If the pilot burner orifice is partially clogged or the thermocouple bracket has shifted, the tip may be outside the flame and generating minimal voltage even on a new thermocouple.
- 3
Dirty Pilot Orifice
A partially blocked pilot orifice produces a weak, unstable flame that doesn't consistently heat the thermocouple to operating temperature. Dirt, rust particles, or spider webs in the pilot tube are common causes. A small blue flame that flickers or has yellow tips instead of a solid cone indicates a dirty orifice.
- 4
Draft or Air Currents
Strong air currents near the water heater — from nearby HVAC vents, an open door, or improper venting — can repeatedly extinguish the pilot. If the pilot goes out only in certain seasons or weather conditions, check for air movement near the base of the heater and ensure the combustion air openings aren't obstructed.
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Quick DIY Checks
Always turn the gas valve to the OFF or PILOT position and allow the pilot assembly to cool completely before handling the thermocouple. Work in a well-ventilated area and verify with your nose that there's no residual gas smell before relighting.
- 1With the pilot lit, look closely at the thermocouple tip — it should be positioned directly in the pilot flame, glowing orange-red. If it's outside the flame or only partially in it, bend the thermocouple bracket slightly to reposition the tip deeper into the flame.
- 2Test thermocouple output with a multimeter on the DC millivolts setting. Attach the positive lead to the thermocouple output terminal on the gas valve and the negative lead to the valve body (ground). A reading below 15mV with the pilot lit confirms the thermocouple needs replacement.
- 3Replace the thermocouple: turn gas valve to OFF, allow the pilot assembly to cool, unscrew the thermocouple compression nut from the gas valve (counterclockwise), remove the thermocouple from the pilot bracket, and install the new universal thermocouple in reverse. The entire replacement takes 15 minutes.
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Try Pro — $7.99/mo- 4Clean the pilot orifice with a short burst of compressed air if the pilot flame looks weak or yellow-tipped. Do not use any metal objects to clean the orifice — it damages the precision opening.
- 5Check for drafts near the water heater base. Hold a lit stick of incense or a piece of tissue near the combustion air opening — if it blows strongly toward the heater, locate and block the air source or install a draft hood.
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Repair vs Replace
A thermocouple replacement is one of the best-value repairs in home plumbing — a $12–$20 part that takes 15 minutes completely resolves the issue. Even if a follow-up pilot assembly or gas valve repair is needed, total repair cost rarely exceeds $150 on a functional tank. Only consider replacement if the tank itself is over 10 years old, has corrosion, or has multiple concurrent failures.
Est. Repair Cost
$15–$30 thermocouple replacement
Est. Replacement Cost
$600–$1,800 for new water heater installed
Recommended Tools & Parts
- Buy on Amazon →
Universal Thermocouple
24-inch universal fit thermocouple that works with most gas water heaters, furnaces, and boilers. The most common pilot outage fix.
$12–$20
- Buy on Amazon →
Thermocouple/Pilot Assembly Combo Kit
Complete pilot burner assembly with thermocouple and pilot tube combined — use when individual thermocouple replacement doesn't resolve the issue or pilot bracket is damaged.
$25–$45
- Buy on Amazon →
Digital Multimeter
Essential for testing thermocouple millivolt output. Any basic multimeter with a DC millivolts setting works for this test.
$15–$30
Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.
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Read guide →Water Heater Pilot Light Won't Stay Lit: Thermocouple Fix
Water heater pilot light keeps going out? A failed thermocouple is the #1 cause — a $15 part you can replace yourself in 30 minutes.
Read guide →Save $150+ on a single service call
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- ✓ Expert diagnosis in seconds — 500+ problems covered
- ✓ Full tool list & cost estimate before you spend a dime
$150+ service call vs. $7.99/mo · Cancel anytime
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Frequently Asked Questions
- How long should I hold down the pilot button when relighting?
- Hold the pilot button down for a full 30–60 seconds after the pilot lights before slowly releasing it. This gives the thermocouple time to heat up and generate sufficient millivolts to hold the gas valve open. If you release too quickly — even with a good thermocouple — the pilot will go out because the thermocouple hasn't reached operating temperature yet.
- Is it safe to replace a thermocouple myself if I've never worked on gas appliances?
- Yes — thermocouple replacement doesn't involve opening any gas lines and carries minimal risk if done correctly. You're simply unscrewing one component and screwing in another. Turn the gas valve to OFF, wait 5 minutes, replace the thermocouple, and test. If you're uncomfortable working near gas appliances, a plumber can do this repair for $75–$150 in labor.
- I replaced the thermocouple and the pilot still won't stay lit — what's next?
- If a new thermocouple doesn't fix it, the gas valve itself has likely failed. Test the new thermocouple with a multimeter to confirm it's generating 20–30mV with the pilot lit. If millivolts are good but the pilot still drops out, the gas valve solenoid is defective and needs replacement — typically $80–$150 for the part, or $200–$350 installed by a plumber.