Gas water heater pilot won't stay lit? Safe checks first, then the thermocouple

Last verified: October 23, 2026

Disclosure: Some links on this site may be affiliate links; see our disclosure. Not affiliated with A.O. Smith, Bradford White, or any water heater maker. Safety first: if you smell gas, don't try to light any appliance, don't touch electrical switches, and don't use a phone in the building. Leave and call your gas supplier from a neighbor's phone; if you can't reach them, call the fire department. [3] Gas valve, thermocouple, and burner work is for a qualified technician.

Two versions of this problem come up constantly: the pilot lights but goes out the moment you let go of the knob, or it stays lit for a while and then keeps going out every few days. Here's what the manufacturers say causes each, what a homeowner can safely check, and what to hand to a pro.

Step 1: Relight it the right way, once

Use the lighting instructions on the label on your heater. A.O. Smith says those instructions supersede any general guide, including this one. [1] Its general steps:

Don't continue if you smell gas. [1]
Turn the gas control knob to OFF and wait at least 10 minutes so built-up gas can disperse. [1]
Set the knob to PILOT and push it in fully, then light the pilot as your label describes. On models with an igniter button, A.O. Smith says to click it about once per second, for up to 90 seconds, while you hold the knob in. [1]
Slowly release the knob and watch that the pilot stays lit, then turn the knob to ON or your temperature setting. On models with a status light, A.O. Smith says it must flash once per second for the heater to work. [1]

If it goes out again right away, don't keep cycling it. Work through the checks below.

Step 2: Why pilots go out (and what you can check)

Flammable vapors nearby. Both manufacturers warn not to store or use gasoline, paint thinner, or other flammable vapors near the heater. [2][3] A.O. Smith's service handbook says its FVIR heaters have a resettable thermal switch that shuts off the gas after high combustion-chamber temperatures from flammable vapors, a blocked vent, or not enough combustion air. [2] Look around for gas cans, solvents, or spills, and move them well away.
Not enough combustion air. A.O. Smith lists lack of combustion air as a cause, for example a heater squeezed into a too-small closet or boxes stacked around it. It warns this can release unburned fuel or carbon monoxide, and says to bring in a tech before relighting if you suspect it. [1] Clear the area around the heater.
A dusty or linty air intake. A.O. Smith's handbook says to check the base-ring filter (the screen around the bottom) for dust and debris at least once a year, and more often if the heater sits where there's lint and dirt, like next to a dryer. With the heater off and cooled for 10 minutes, it can be cleaned with a vacuum hose attachment. [2] If you can't reach or see the filter, leave the inner flame arrestor cleaning to a technician; that means pulling the burner assembly. [2]
Drafts. A.O. Smith says a draft, even an open garage or utility-room door, can blow out the small pilot flame. [1]
Gas supply problems. A.O. Smith lists gas pressure changes (which may point to a problem outside the heater) and kinked or clogged tubing between the gas supply and the control. [1] If other gas appliances are acting up too, call your gas utility.

Step 3: Goes out when you let go of the knob? Usually the thermocouple

A.O. Smith explains that the thermocouple or thermopile powers the gas valve. If it's bad, "the pilot will not remain lit once the pilot knob is released." [1] Its service handbook adds that it can take up to 180 seconds for the thermocouple to cool enough to close the safety valve after the pilot goes out. [2] That's one more reason to wait before you relight.

What a technician will typically test, in the order the manuals give:

Thermocouple output. On the A.O. Smith residential FVIR models covered by its handbook, a meter should read 20–30 millivolts DC or more after 45 seconds. If it doesn't, the tech checks that the pilot flame touches the thermocouple tip, then replaces the thermocouple if it still fails. [2] Some heaters use a thermopile instead, with different numbers. The Bradford White service manual for its Atmospheric ECO-MAGNUM models gives its own thermopile test. [3] Use the values in your model's manual; they aren't interchangeable.
Pilot assembly. Bradford White's manual has the tech inspect the pilot's primary air openings for debris "(dirt, lint, etc)," look for kinks or cracks in the pilot tube, and clean or replace a blocked pilot orifice. [3]
Thermal switch and gas control. If the thermal switch tripped, A.O. Smith's procedure is to find the cause (lack of combustion air or a flammable vapor event) before resetting it. If the pilot still won't stay lit, the switch or the gas control valve is replaced. [2] A.O. Smith says not to disable the thermal switch. [2] Bradford White also says the overheating cause must be found before the heater goes back in service. [3]

When to stop and call

You smell gas (rotten eggs or sulfur): leave and call your gas supplier. Bradford White notes the odorant can fade, so if you're unsure, call anyway. [3]
The thermal switch keeps tripping, there's soot or scorching, or you suspect a vent or air supply problem. [1][2]
A.O. Smith: if the pilot won't cooperate, find a local service provider. If repairs fail, it may be time for a new unit. [1]

Have your model and serial number (on the rating plate) ready, so you get the right manual and parts.

Other no-heat guides

Oven cold but the burners work? See gas oven not heating. Dryer: Whirlpool · Samsung. In an RV, the water heater is a different system: see RV furnace and water heater not working.

FAQ

Is it dangerous if the pilot goes out? A.O. Smith says built-in safety features shut off the gas flow when the flame goes out, but never ignore a gas smell. [1]

Why wait 10 minutes before relighting? A.O. Smith says the wait lets built-up gas disperse so you don't ignite it. [1]

Can I replace the thermocouple myself? The A.O. Smith and Bradford White procedures are written for licensed or qualified service providers. [2][3] Because it means disconnecting gas components, we recommend a pro.

We don't list part prices, repair costs, or millivolt values for models other than those covered by the cited manuals. Check your own heater's label and manual.

Sources

General diagnostic guidance only, not specific to any single model. Sourced from the A.O. Smith and Bradford White pages and manuals linked above. Last verified October 23, 2026.