Water Heater Pilot Light Won't Stay Lit: Causes, Safe Checks, and When to Call a Pro

On a gas tank water heater with a standing pilot, the pilot is a small flame that burns around the clock and lights the main burner when the water needs heat. A.O. Smith explains that a thermocouple or thermopile, energized by that flame, powers the gas valve; when the pilot goes out, gas to the pilot stops and the heater shuts off. So a pilot that won't light, goes out when you release the knob, or dies hours later is a safety system at work; the job is finding out why.

This page covers gas tank heaters. For an electric heater or a tankless unit, see no hot water; if the water heats but runs out early, see hot water runs out fast. If an inspector flagged exhaust spilling at the vent, see water heater backdrafting.

Start Here: What Is the Pilot Doing?

• It won't light at all. A.O. Smith's troubleshooting chart lists the gas control knob in the wrong position, the main gas supply off, a failed thermocouple, and, on push-button models, no spark. A tripped flammable vapor safety (cause 3) also stops it.
• It lights, then goes out when you release the knob. A.O. Smith says a broken thermocouple means the pilot "will not remain lit once the pilot knob is released." State's flowchart starts from this symptom and checks the thermal switch, its wiring, and the thermocouple.
• It stays lit, then goes out later. Look at the air around the heater: drafts, a clogged air screen, condensation dripping on the burner, or summer heat in an attic.

Rheem's pilot outage bulletin offers a clue: if the heater worked fine for more than a few weeks and the outages only just started, gas supply and combustion air in the installation are less likely to be the problem.

Relight Only by the Label

Read your heater's own steps before you touch the knob. Two examples show how much they vary:

• A push-button igniter. State's label for its White-Rodgers control says to turn the knob off and wait 10 minutes to clear out any gas; if you then smell gas, stop. After the pilot lights, keep holding the knob down for about one minute. If the pilot "will not stay lit after several tries," turn the knob to OFF and "call your service technician or gas supplier."
• Automatic ignition. A.O. Smith's label for a heater that lights its own pilot says "DO NOT TRY TO LIGHT THE PILOT BY HAND." If it fails to light after several tries, turn off the gas to the heater, call a qualified service technician or the gas supplier, and record any status light flash codes.

On flammable-vapor-resistant heaters, State's label says not to open the inner door and try to light the pilot by hand. For propane, the Propane Education & Research Council says to follow the appliance maker's instructions, and that if a pilot is difficult to light or repeatedly goes out, never try to fix or troubleshoot it yourself.

Most Likely Causes

1. A worn or out-of-place thermocouple (or thermopile). Rheem's bulletin says the thermocouple's tip must sit in the pilot flame. A.O. Smith's chart lists a failed thermocouple and a tip not touching the flame; State's adds a loose thermocouple connection at the gas control. Some heaters use a thermopile instead: on Bradford White's ICON control, two flashes and a pause mean low thermopile voltage, from loose connections or a weak pilot flame. A.O. Smith wrote its chart for a qualified service technician, and the fix there is a new pilot assembly or thermocouple.
2. A dirty pilot or a clogged pilot tube. A.O. Smith's chart lists a dirty pilot burner. State's lists a clogged pilot line or orifice, both for a pilot that won't light or stay lit and for a pilot flame that's too small. Rheem's bulletin has the technician remove and clean the pilot orifice and pilot hood. A.O. Smith also names a kink or clog in the flexible tubes between the fuel supply and the gas control. All of this is technician work.
3. A tripped flammable vapor safety, or a clogged air screen. The CPSC says a voluntary standard calls for conventional tank-type gas water heaters manufactured after July 1, 2003, to carry a flame arrestor, which traps and burns flammable vapors inside the heater instead of letting them ignite in the room; it covers 30-, 40-, and 50-gallon models. Rheem's, State's, and A.O. Smith's manuals call these heaters FVIR, for flammable vapor ignition resistant. Rheem calls keeping the flame arrestor from clogging with lint, dust, and oil a challenge for every manufacturer. State says its heaters shut off when dust builds up on the base-ring filter or flame arrestor, and a thermal switch cuts the gas after flammable vapors, a blocked vent, or too little combustion air. Some Rheem and A.O. Smith models also use a flammable vapor sensor that locks the heater out and flashes a code. Resets differ: State's manual describes a reset button on the thermal switch for use once the cause is corrected, while A.O. Smith and Rheem send a vapor sensor lockout to a qualified service technician.
4. No gas, or not enough. A.O. Smith's chart lists the main gas supply turned off, and its shutoff valve is open when the handle lines up with the gas pipe. A.O. Smith says the pilot is sensitive to gas pressure changes, and an ongoing problem usually lies outside the heater, perhaps in the external gas lines. State's chart sends no gas, low gas pressure, and dirt in the gas lines to the gas utility. On propane, PERC says an appliance that isn't working may mean you're out of gas, and Bradford White warns that running out can damage the heater.
5. Drafts, wind, or too little air. A.O. Smith says that in a garage or a utility room that opens outside, an open door can let in a gust that puts out the pilot. Rheem's bulletin adds drafts down the vent, a ceiling fan in the same room, and drafts under a door; on direct-vent models, wind blowing into the outside vent can cause nuisance outages. State warns that exhaust fans, dryers, and fireplaces can reverse the flow of flue gases and raise carbon monoxide indoors; that's backdrafting, and a technician tests for it. Rheem also says attic heaters are prone to pilot outages in summer heat, and one in a cramped closet may not get enough air. A.O. Smith warns that too little air can release unburned fuel or carbon monoxide, so bring in a technician before relighting.
6. A failed gas control valve. A.O. Smith says the control valve can fail over time, and then it will most likely need to be replaced on a service visit. Rheem's bulletin says there's no field test for the valve, so it's suspected once the other checks pass. State and Rheem add that if the single-use high-limit cutoff (ECO) trips, the whole gas control must be replaced. A.O. Smith says the valve "should be inspected and replaced by a professional if found to be defective."
7. Condensation on the burner. A.O. Smith and State both say: "Excessive condensation can cause pilot outage due to water running down the flue tube onto the main burner and putting out the pilot." State says it usually happens when a new heater is first filled with cold water or a lot of hot water is used fast and the refill is very cold, shows up most in winter and early spring, and should stop once the tank warms up, in about one to two hours. If water keeps showing up after the tank is hot, see water heater leaking.

What You Can Fix Yourself

• Check the gas shutoff at the heater: open when its handle lines up with the pipe, per A.O. Smith. If it's closed and you don't know why, call before opening it. Our shutoffs guide shows where it usually is.
• Check your other gas appliances. If the range and furnace won't light either, the problem is the supply: call the gas utility, as State's chart says for no gas, or on propane, your propane retailer. PERC says that after a run-out a qualified professional must leak-check the system when the gas goes back on; never turn it on at the tank yourself, and never try to let air out of the gas line, which releases gas.
• Stop the drafts. Close the door, turn off a nearby fan, and clear boxes and clutter from around the heater, which A.O. Smith says can starve the pilot of air.
• Keep the air openings clean. A.O. Smith's manual says to vacuum dust, dirt, and lint from around the base regularly and not to block the combustion air openings there. State's routine maintenance includes vacuuming the base-ring filter at least yearly, after turning the heater off per its lighting instructions and letting it cool 10 minutes. Clean a filter or screen only if your manual says owners may.
• Relight once, by the label, if you don't smell gas, and write down any status light code.

When to Call a Pro

Call a licensed plumber or gas technician, or your gas utility for supply problems, when:

• The pilot won't stay lit after several tries. Rheem says a plumber can safely get to the root of the cause and fix it.
• A thermocouple, thermopile, pilot assembly, or gas valve needs replacing. State says its FVIR component procedures should be performed by a qualified technician, and Bradford White says not to attempt repair of the combination gas control, burners, or gas piping.
• Any gas-line work, including a suspected leak, a kinked tube, or restarting propane service.
• A flammable vapor lockout or thermal switch trip comes back, or you see blue or black discoloration on the flame arrestor. If vapors may have ignited, State says not to use the heater: it needs replacing.
• You see soot or the heater sits in a tight space. A.O. Smith says soot requires correction before further use; turn the gas to the heater off until it's repaired.

Have the model and serial number and any flash code ready for the visit.

Prevent It

• Keep flammables away. Bradford White says never to use or store gasoline or paint thinner near the heater or anywhere fumes could reach it. The CPSC says gasoline "should never be stored or used indoors (in a basement or garage) where vapors can ignite."
• Book a yearly check. Bradford White says a qualified service technician should inspect the heater at least once a year.
• On propane, check the tank gauge periodically, as PERC suggests.
• Put CO alarms on every level and outside sleeping areas, as the CPSC recommends. See our smoke and CO alarm guide, and if one sounds, what to do when a CO alarm goes off.

Frequently Asked Questions

Why does my pilot go out as soon as I let go of the knob?

Most often the thermocouple or thermopile: worn, out of the flame, or loosely connected, or a weak pilot flame, according to A.O. Smith, State, and Bradford White. Hold the knob for the time your label gives. If it still goes out after several tries, turn it off and call a technician.

Can I replace the thermocouple myself?

The makers put that job with a technician: State reserves its FVIR component procedures for a qualified technician, A.O. Smith writes its troubleshooting chart for one, and Bradford White says not to attempt repairs to the gas control or burners.

Is it dangerous when the pilot goes out?

A.O. Smith says a pilot going out "is generally safe" because built-in safety features shut off the gas when the flame goes out. A gas smell, soot, or a CO alarm changes that; follow the steps in the box at the top.

How do I know whether the flammable vapor system tripped?

State says to check the flame arrestor for blue or black discoloration. On Rheem and A.O. Smith models with a vapor sensor, seven flashes mean a flammable vapor sensor lockout; A.O. Smith says to remove gasoline, paint, thinners, varnish, and cleaners from the area and call for inspection.

The Bottom Line

If you smell gas, leave. Otherwise, confirm the gas is on, stop drafts, keep the base clean, and relight once by the label. If it won't stay lit, the thermocouple, pilot, gas valve, and flammable vapor parts are a technician's job.

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