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

Unique angle here: a compact cause-and-cost list. The longer service-call diagnosis is Water Heater Pilot Light Won’t Stay Lit.

Desk note: Costs below are hardware-store and service-call research ranges, not quotes. This is a cause list from manufacturer relight labels and common field write-ups, not a first-person service diary. A pilot that dies when you let go of the knob is a thermocouple-first problem. It is not proof the tank is finished.

Smell gas with the control off? Leave the house and call the gas company. That is not a DIY relight and not a parts order.

If the tank is also rust-tinting the hot side, or a drain valve is weeping, those are separate pages: DIY vs plumber and why hot water is brown or rusty. Relighting a pilot does not restore an anode.

Before You Buy a Thermocouple

Smell test. Gas odor with the knob off is an emergency call, not a hardware-store run.
Read the relight label on the jacket. Turn to OFF, wait the printed minutes, then PILOT, hold, ignite, hold 30–60 seconds, then ON. Do that once, by the label, before you assume the part is dead.
Watch what the flame does. Lights while you hold, dies when you release: thermocouple or its position in the flame. Lights and then blows out later: draft or a dirty pilot tube. Never lights: supply, igniter, or a closed gas cock.
Measure the old lead if you are replacing it. Length from the pilot bracket to the gas-valve port, and look at the nut. Search pages mix furnace hot-surface igniters with water-heater millivolt thermocouples. Tagged thermocouple search is safer than guessing an ASIN.
Then decide part vs plumber. A thermocouple is a parts-bin fix when you can reach the valve and you do not smell gas. A gas-control valve on an old tank is a replace-the-heater conversation.

Listing check (pilot compact list): confirm a millivolt water-heater thermocouple whose lead matches the old length — 24-volt furnace hot-surface igniters and 48-inch furnace-only leads share the same browse page.

The other sibling with the troubleshooting title is pilot light won’t stay lit: troubleshooting. Fuel choice if you are done with standing pilots: electric vs. gas.

How the Pilot Light System Works

The pilot is a small flame that lights the main burner when the thermostat calls for heat. A thermocouple sits in that flame and holds the gas valve open with a small millivolt signal. If the pilot goes out, the thermocouple cools and the valve closes. That is the safety, not a defect.

Cause 1: Dirty or Bent Thermocouple

The usual cheap cause. Soot, carbon, or a tip sitting beside the flame instead of in it will drop the millivolt signal and close the valve.

Repairable: power/gas to PILOT or OFF per the label. Cool. Clean the tip with fine abrasive. The tip should sit in the flame, not beside it. A gentle bend is enough; do not kink the copper.

Research cost: $0 if cleaning holds.

Cause 2: Failed Thermocouple

If cleaning and position do not hold the pilot after you release the knob, replace the thermocouple. Universal water-heater leads commonly list in a $8–$20 research range. Lengths are often 24, 30, or 36 inches. Match the old one.

Repairable: shut gas, remove the old lead from the valve and the bracket, install the new one in reverse, then relight by the jacket label.

Research cost: $8–$20 DIY parts, or a $80–$150 service-call research range if you hire it.

Cause 3: Dirty Pilot Tube

The small tube that feeds the pilot can clog with dust, lint, or insect debris.

Repairable: gas off. A thin needle or dry compressed air at the orifice, not a shop solvent dump into the valve. Relight and watch the flame: it should be steady and wrap the thermocouple tip.

Skip DIY if you cannot isolate the gas or you are working next to an unknown leak.

Cause 4: Drafts

A basement window, a garage door, or an HVAC return can blow the pilot out after a successful relight. The flame looks fine until the door opens.

Repairable: find the draft. The draft hood on an atmospheric tank must stay clear — do not box it in. Closing a window is a diagnosis, not a permanent “fix” if the vent is backdrafting. Backdrafting or flue spillage is a plumber / HVAC call, not a longer thermocouple.

Cause 5: Faulty Gas Valve

If the thermocouple is new, the tube is clear, and drafts are ruled out, the gas control itself may be the part.

Replace-the-part vs replace-the-tank: gas-valve replacement is commonly a $200–$400 research range installed. On a tank already in the published 8–12 year window, or a tank with a wet shell, that money usually belongs on a heater, not a valve. That decision is DIY vs plumber.

Not DIY: gas-valve work, any smell of gas, or a control you cannot identify on the rating plate.

Skip DIY, Call the Gas Company or a Plumber

Gas odor, hissing at the control, or a relight that will not stay after you have left the area
Flue spillage, soot on the jacket, or a draft hood that is spilling
A gas valve on an old tank when the shell is wet or the anode is gone
You cannot reach the assembly without lifting the tank or cutting the vent

A working gas tank still needs sediment maintenance. After the pilot holds, use the compact hose-and-valve flush if the drain runs. More symptom pages sit on the troubleshooting hub.

Sources

Manufacturer relight labels and service sheets (Honeywell / White-Rodgers / A.O. Smith style) for hold-time and thermocouple-first order
U.S. Department of Energy, Water Heating, gas water-heater safety context

Safety Disclaimer