Water Heater Pilot Light Won't Stay Lit: Causes and Fixes

You light the pilot, hold the knob down like the label says, watch a steady blue flame, count to sixty, and let go. The flame dies. You do it again and get the same result. That single symptom, a pilot that burns only while you press the knob, has a short list of causes, and they run from a five-dollar part to a gas valve replacement. The trick is reading the flame and testing the sensor before you throw parts at it. Almost every case traces back to one question: is the thermocouple making enough millivolts to hold the safety magnet, or is something upstream, a dirty pilot, a starved FVIR screen, a soft lockout, keeping it from ever getting hot enough.

How the safety chain actually works

Every standing-pilot gas water heater has the same fail-safe: no pilot flame, no gas. The pilot flame heats a thermocouple, a thin metal probe sitting right in the flame. The heated junction makes a small DC voltage, tens of millivolts, and that voltage runs down the copper lead to an electromagnet inside the gas control valve. That magnet, sometimes called the pilotstat, is what physically holds the pilot gas valve open. When you press the knob in during lighting, you are doing the magnet's job by hand, forcing the valve open so the pilot can burn. The 30 to 60 seconds you hold is the thermocouple warming up to full output. Let go, and the magnet has to hold the valve on its own. If the voltage is there, it holds. If not, the spring shuts the valve and the flame dies.

That is why the symptom is so specific and so useful. A pilot that will not even light points at gas supply, a clogged orifice, or an FVIR lockout. A pilot that lights, burns while held, and dies on release points squarely at the thermocouple-to-magnet circuit. Same flame, very different repair. Reading which one you have saves you from swapping parts blind.

1. Watch the pilot flame while it is lit

   It should be a steady blue flame that fully engulfs the top half inch of the thermocouple tip. Small, orange, floppy, or lifting off means a dirty orifice, a fouled tube, or a draft. A flame that misses the tip cannot heat the sensor no matter how long you hold the knob.

2. Confirm you held the knob long enough

   Relight and hold a full 60 seconds this time. A surprising share of no-hold complaints are just a knob released at 15 seconds before the thermocouple reached temperature. Rule this out first.

3. Test the thermocouple millivolts

   With a multimeter on millivolts DC, a heated thermocouple reads about 25 to 35 mV open-circuit. Near 20 mV or below, replace it. Under load it should still hold above the roughly 2 to 5 mV drop-out point; a reading that collapses under load is a tired sensor or a weak flame.

4. Rule out an FVIR lockout

   If the pilot will not stay lit and the flame looks starved or lazy, check the flame-arrestor screen at the base for dust and lint, and clear any solvents or gasoline from the room. A soft lockout ignores a good thermocouple entirely.

5. Suspect the valve magnet last

   If you install a new thermocouple, the flame is perfect, and it still dies on release, the electromagnet in the gas valve has failed. That is a valve replacement, not a DIY part.

The thermocouple, the part that fails most

Nine times out of ten a pilot that will not stay lit is the thermocouple, and it fails in one of three ways: it is dirty, it is out of position, or it is simply worn out. Soot builds on the tip over years and insulates it from the flame, so the same pilot that used to make 30 millivolts now makes 15. The probe can also get bumped out of the flame during service, so the blue cone no longer wraps the tip. And metal fatigue is real; after a decade of heat cycles the junction just stops producing. Cleaning the tip with fine emery cloth and nudging it back into the flame fixes the first two. The third needs a new one, which is a cheap part and a common repair. A thermocouple runs a few dollars, threads onto the valve with a simple nut, and the swap takes about fifteen minutes once the burner assembly is out. There is no reason to pay a service call for the part itself; the labor is the only real cost, and it is small.

The way to know for sure is the millivolt test. Pull the thermocouple lead off the valve, put a multimeter on the DC millivolt range across the tip and the copper, and light the pilot. A good one climbs to about 25 to 35 millivolts within a minute. If it never gets past 20, or it reads fine open but crashes when you reconnect it under the load of the magnet, that sensor is the problem. The magnet needs the voltage to stay above roughly 2 to 5 millivolts to hold, so a sensor that sags below that under load will always drop the flame on release.

When it is not the thermocouple

Before you decide the sensor is bad, look at what feeds the flame. A pilot orifice clogged with dust, lint, or a spider web makes a starved flame that cannot heat any thermocouple, and no amount of sensor swapping fixes it. Shut off the gas, let it cool, and blow the pilot tube and orifice clear with compressed air. Do not ream the orifice with wire; widening that hole changes the flame and you will chase it forever. Weak gas supply does the same thing from further upstream. A half-closed supply valve, a nearly empty LP tank, or a regulator problem all shrink the pilot. If several gas appliances act starved at once, the problem is supply, not the water heater.

FVIR: the modern lockout most people miss

Any gas water heater built since 2003 is FVIR, flammable vapor ignition resistant, and it adds a failure mode older tanks never had. The unit draws combustion air through a fine flame-arrestor screen at the base, and it carries a sensor that watches for flammable vapors so a spill of gasoline, solvent, or paint thinner nearby cannot flash back into the burner. When that system decides something is wrong it goes into a soft lockout, and the pilot will not stay lit no matter how perfect your thermocouple is. People replace two thermocouples chasing this before they realize the sensor, not the flame, is calling the shots.

Clearing the trigger comes before any reset. Get solvents, gasoline, and fresh paint out of the room, ventilate, and vacuum the arrestor screen. The reset itself depends entirely on the brand. Some units, like Bradford White's Defender, reset with a knob or button sequence and no new parts. Others use a one-time thermal cutoff switch or a vapor sensor that has to be replaced once it trips, especially if it got wet from a nearby water softener discharge. Read the label on your control and the manual for the exact procedure. Natural gas and LP units differ in orifice size and settings, so match every step to the manual for your specific model rather than a generic video.

The gas valve magnet, and where you stop

The last suspect is the one you confirm by elimination. If the pilot flame is clean and blue and wraps the tip, a millivolt test shows the thermocouple making a healthy 30 or so, there is no FVIR lockout, and it still dies the instant you release the knob, the electromagnet inside the gas valve has failed. The magnet can no longer hold the valve open even with good voltage feeding it. There is no rebuild for it. The gas control valve gets replaced as a complete unit, and because that means breaking into the gas line, it is a job for a licensed plumber or gas tech, not a homeowner afternoon.

• You smell gas near the heater. Leave the building, then call the gas utility from outside. Do not flip switches, use a phone indoors, or light anything.
• A brand new thermocouple still dies on release, which means the gas valve magnet has failed.
• The FVIR reset needs a thermal cutoff switch or vapor sensor replaced on a brand that is not homeowner-resettable.
• The pilot orifice or valve body needs work that opens the gas line.
• Repeated lockouts keep coming back after you have cleaned the screen and cleared vapors.
• You are not comfortable shutting off the gas and following the burner-door lighting legend.

Everything above the gas line is fair homeowner territory with the control off and the legend open: holding the knob the full minute, cleaning the pilot and thermocouple, testing millivolts, vacuuming the FVIR screen, resetting a brand that allows it. Everything that opens the valve body or the gas supply is where a tech earns the call, because the failure mode is a gas leak or a combustion problem. When in doubt, read the flame first. It tells you more than any parts guess.