Water Heater Pilot Light Out — How to Relight and What to Check If It Won't Stay Lit

When a gas water heater pilot light goes out, the burner can't ignite and the water stays cold. The fix is often as simple as relighting the pilot — a 10-minute free repair. But if the pilot won't stay lit after you hold the knob for 30–60 seconds, the thermocouple is almost certainly the problem. This guide walks through relighting, diagnosing a failed thermocouple, testing it with a multimeter, and knowing when to call your gas company instead.

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Common Symptoms

No hot water from any tap — only cold water
Water starts warm then goes cold quickly
Pilot light is not visible through the sight glass on the water heater
No burner ignition sound when the thermostat calls for heat
Smell of gas around the water heater (emergency — do not relight, call gas company)

Most Likely Causes

1
Pilot Light Blown Out by Drafts or Combustion Air Disturbance
Pilot lights can be extinguished by strong drafts, a recently opened or closed door nearby, negative air pressure in the mechanical room (common in tightly sealed homes), or a gust entering through the flue during a windstorm. This is the simplest cause — a straightforward relight fixes it completely.
2
Failed Thermocouple (Most Common — Pilot Lights But Won't Stay Lit)
The thermocouple is a thin copper probe with its tip positioned directly in the pilot flame. It generates a small millivolt signal that tells the gas valve the pilot is lit — if it detects no flame, the gas valve snaps shut as a safety measure. When the thermocouple degrades, it generates insufficient voltage (less than 15 mV) and the gas valve cuts off within 30–90 seconds of releasing the pilot knob. Thermocouple replacement costs $10–$25 and takes about 30 minutes.
3
Dirty or Misaligned Thermocouple Tip
Carbon or mineral buildup on the thermocouple tip insulates it from the flame, reducing its millivolt output. Gently cleaning the tip with fine steel wool or emery cloth can restore output. Also check that the tip is positioned directly in the hottest part of the pilot flame (the inner blue cone, not the outer orange tip).
4
Weak or Failing Gas Valve
If the thermocouple tests within spec (15–30 mV with a multimeter) but the pilot still won't hold, the gas valve's internal electromagnet is not holding correctly. Gas valve replacement costs $50–$150 for the part. This is the correct diagnosis only after confirming the thermocouple is good.
5
Flex Gas Supply Line Issue or Low Gas Pressure
A partially closed manual gas shutoff valve upstream of the water heater, or a utility supply pressure drop (during peak demand), can cause the pilot to fail to light or stay lit. Verify the gas shutoff valve is fully open (handle parallel to the pipe on a ball valve) before diagnosing other components.

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Quick DIY Checks

Safety Warning

If you smell gas (rotten egg / sulfur odor) near the water heater, do NOT attempt to relight. Do not operate any switches. Leave the house immediately and call your gas utility's 24-hour emergency line from outside. Gas leaks can cause explosion or asphyxiation.

Safety Warning

Never force the pilot control knob. If it is stuck or requires excessive force, the gas valve is damaged and needs replacement — forcing it can crack internal components and create a dangerous gas leak.

Caution

Wait a minimum of 5 minutes between failed relight attempts. This allows any accumulated raw gas to disperse before you introduce a spark. Even a small gas accumulation near an ignition source is a fire hazard.

Caution

Do not use an open flame lighter or match in an enclosed cabinet or cramped space. Use a long fireplace lighter to maximize distance from your face and the gas supply. Ensure the space is ventilated before lighting.

1First: confirm it's a gas water heater. Electric water heaters have no pilot light — they have heating elements and a reset button. Gas heaters have a gas line connected to them (usually a black iron pipe or flexible corrugated connector), a thermostat knob labeled OFF / PILOT / WARM / HOT, and a draft hood on top leading to a flue pipe. If you see no gas line and no flue, you have an electric unit — this guide does not apply.
2Safety check first — smell for gas. Before attempting any relight, stand near the water heater and smell carefully. If you detect a sulfur (rotten egg) odor, do NOT attempt to relight. Leave the house immediately, leave the door open behind you, do not operate any light switches or electronics, and call your gas utility's emergency line from outside or a neighbor's home. Do not re-enter until the gas company clears it.
3Wait after a failed attempt. If the pilot recently went out or you've had a failed relight attempt, wait at least 5 minutes before trying again. This allows any accumulated gas to dissipate. Good ventilation is important — open a window if possible.

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4Locate the pilot assembly and controls. Find the thermostat dial at the base of the water heater — it has positions labeled OFF, PILOT, WARM, HOT (and sometimes specific degree markings). There will be a red or black igniter button nearby, and the gas valve/thermostat assembly will have a push-down or push-in pilot knob. Locate the sight glass (a small rectangular window, usually in the lower front panel) so you can see the pilot flame.
5Step-by-step relight procedure: (1) Turn the thermostat dial to the OFF position and wait 5 minutes. (2) Turn the dial to PILOT. (3) Press and hold the pilot knob or button firmly down — this opens the pilot gas supply line mechanically, bypassing the thermocouple safety. (4) While holding the knob down, press the spark igniter button repeatedly (once per second) until you see the pilot flame through the sight glass. On units with no igniter button, use a long lighter or long match inserted through the access hole near the pilot tube. (5) Once the pilot is lit, continue holding the pilot knob down for 30–60 seconds — this heats the thermocouple so it generates enough voltage to hold the gas valve open. (6) Slowly release the pilot knob. If the pilot stays lit, the repair is complete. (7) Turn the thermostat dial to your desired temperature (120°F is recommended). The main burner should ignite within a few minutes.
6Brand-specific notes: Rheem and Ruud units built after 2010 use a piezo igniter button (round red button) located on the side of the thermostat assembly — some older models require a separate igniter probe inserted through an access hole. Bradford White units may have a standing pilot (ignite with a lighter) or electronic ignition (spark igniter built in) — check the label on the front of the unit. A.O. Smith models with an Intellivent or FVIR system use a blue igniter button and the pilot knob is an integral part of the gas valve — do not force it; press firmly and steadily.
7Pilot won't stay lit after 60 seconds — diagnose the thermocouple. If the pilot lights but goes out as soon as you release the pilot knob (or within 30–90 seconds), the thermocouple has failed or needs replacement. Visual check: locate the thermocouple — it's a thin copper rod about 1/4 inch in diameter, with its tip positioned in the pilot flame and the other end connecting to the gas valve via a threaded fitting (3/8-24 thread on most residential units). The tip should be in the hottest part of the flame. If the tip looks white, coated, or corroded rather than bright copper, it likely has a buildup problem — try cleaning it gently with fine steel wool before replacing.
8Test the thermocouple with a multimeter: turn the pilot on and hold the pilot knob down while a helper holds a multimeter set to millivolt (mV) mode with the leads touching the thermocouple connector terminals. A good thermocouple generates 15–30 mV with the pilot flame fully heating it. A reading below 12 mV indicates a failing thermocouple. A reading of 0 mV means it has completely failed. Note: test while the pilot is actively burning and you've held the knob for at least 30 seconds.
9Replace the thermocouple (DIY — 30 minutes): shut off the gas supply valve fully, allow 5 minutes for residual gas to clear. Remove the access panel. Unscrew the thermocouple from the gas valve (usually a 3/8-24 thread — a 7/16-inch open-end wrench fits most). Disconnect the thermocouple tip from the pilot bracket clip — it pulls out or unscrews depending on model. Take the old thermocouple to a hardware store for a matching replacement (most are universal 24-inch or 36-inch lengths — use the same length or longer, never shorter). Reinstall in reverse order: clip tip into pilot bracket with tip in the flame path, hand-tighten then 1/4 turn snug on the gas valve fitting (do not overtighten — these are soft copper fittings). Relight the pilot and verify it stays lit.
10If the thermocouple tests good (above 15 mV) but pilot still won't hold — gas valve replacement. The internal electromagnet (also called the thermopile solenoid) in the gas valve may be faulty. Gas valve replacement is a $50–$150 part (brand-specific — search by model number). The job involves disconnecting 3 gas lines (main burner, pilot, and manifold) and 2–3 wiring connectors. While it is a DIY-capable repair, many homeowners prefer a licensed plumber for any work involving disconnecting gas lines. Professional installation runs $200–$400 total.

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Repair vs Replace

✓ Worth Repairing

A pilot light out is almost always worth repairing — the fix is free or under $25 in most cases. Even a gas valve replacement ($50–$150 part) is worthwhile on a unit under 10 years old. Only consider replacement if the water heater tank is leaking, the unit is over 12 years old with multiple issues, or you want to upgrade to a tankless or heat pump water heater.

Est. Repair Cost

$0 to relight; $10–$25 for thermocouple; $50–$150 for gas valve (installed $200–$400)

Est. Replacement Cost

$900–$1,800 for a new gas water heater (installed)

Recommended Tools & Parts

Universal Water Heater Thermocouple (24-inch or 36-inch)
Replacement thermocouple for gas water heaters. 3/8-24 thread fitting fits most residential brands (Rheem, A.O. Smith, Bradford White, State, American). Buy the same length as the original or longer. Premium units include a longer lead and universal pilot bracket clips.
$10–$25
Buy on Amazon →
Gas Control Valve / Thermostat Assembly
Replacement gas valve and thermostat assembly for gas water heaters. Must match brand and model number exactly — search by your unit's model number on the rating plate. Includes pilot tube, thermocouple port, and main gas outlet.
$50–$150
Buy on Amazon →
Long Fireplace Lighter
Extended-reach lighter for safely igniting pilot lights in tight spaces without getting your hand near the gas supply. Essential for water heaters without a built-in piezo igniter.
$5–$12
Buy on Amazon →
Digital Multimeter
Used to test thermocouple millivolt output (mV setting). A reading of 15–30 mV with pilot lit indicates a good thermocouple; below 12 mV means replace it.
$15–$30
Buy on Amazon →

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

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Frequently Asked Questions

How long should I hold the pilot knob down?: Hold it down for a full 30–60 seconds after the pilot ignites. The thermocouple needs time to heat up to the temperature at which it generates enough millivoltage to hold the gas valve open. Releasing before 30 seconds is the most common reason the pilot immediately goes out. In cold utility rooms, hold for the full 60 seconds.
My pilot lights but goes out as soon as I release the knob — is this definitely the thermocouple?: It's the thermocouple in roughly 80% of cases. Confirm by checking: is the thermocouple tip positioned in the pilot flame? Does the tip look clean (bright copper), or white/corroded? If it's dirty, clean it first. If it's clean and properly positioned and still won't hold, replace the thermocouple — it's the correct and inexpensive first step.
Can I replace a thermocouple myself if I'm not a plumber?: Yes. The job requires no gas line disconnection — the thermocouple connects to the gas valve with a compression fitting that unscrews with a wrench, and to the pilot bracket with a simple clip. The gas supply valve stays closed throughout. It's one of the simplest gas appliance repairs and typically takes 20–30 minutes.
How do I know if my water heater is gas or electric?: Gas water heaters have a gas supply pipe (black iron or flexible corrugated stainless) connected to the bottom of the thermostat assembly, a thermostat/gas valve knob with OFF/PILOT/HOT settings, a draft hood on top connected to a flue pipe, and a combustion air opening at the bottom. Electric water heaters have a 240V cable entering from the top or side, two thermostat access panels on the side of the tank, and no flue pipe.