How the pilot and gas valve work together
On a traditional gas tank, a small pilot flame burns all the time. Sitting in that flame is a thermocouple, a copper-tipped rod that produces a tiny electric current when it is hot. That current holds a safety magnet open inside the gas control valve. If the pilot goes out, the thermocouple cools within seconds, the magnet lets go, and the valve shuts off all gas to the heater. It is a simple, clever safety system that has been around for decades.
Newer heaters often use a thermopile (a bundle of thermocouples that makes more current) to power an electronic gas valve with a status light. Some use a hot surface igniter instead of a standing pilot. The logic is the same: no proof of flame, no gas.
So when a pilot will not stay lit, the question is whether the flame is going out, or whether the flame is fine and the valve is not getting or not trusting the signal.
The four usual causes
| Cause | What you notice | Typical fix range |
|---|---|---|
| Worn thermocouple or thermopile | Pilot lights while holding the button, dies within a minute of letting go | $150–$350 |
| Dirty or weak pilot flame | Small, yellow, lazy flame that does not wrap the thermocouple tip | $150–$250 |
| Flammable vapor sensor or air intake clogged | Pilot goes out repeatedly; lint or dust around the bottom screen | $150–$400 |
| Failed gas control valve | Pilot will not light at all, or status light shows a valve fault code | $350–$650 |
Since about 2003, most gas heaters sold in the US have a sealed combustion chamber with a flame arrestor screen at the bottom, designed to keep garage fumes from igniting. That screen pulls air through a mesh that clogs with dust, lint and pet hair. When it clogs, the pilot starves for air and goes out. Cleaning it is a common and inexpensive fix, but on some models it means pulling the burner assembly.
Relighting safely
Every gas heater has a lighting label on the side of the tank. Follow that label, not a generic video. The general pattern on a standing-pilot heater:
- Turn the gas control knob to OFF and wait five minutes for any gas to clear.
- If you smell gas at any point, stop, leave the area and call the gas utility from outside.
- Turn the knob to PILOT, press and hold it, and press the igniter button until you see the flame through the window.
- Keep holding for about 60 seconds so the thermocouple heats up, then release.
- If the pilot stays lit, turn the knob to ON and set the temperature dial.
Electronic valves with a status light have their own sequence, often just turning the knob to ON and letting the igniter cycle. If the light flashes a code instead of lighting, count the flashes and check the chart on the label. Two flashes and four flashes mean different parts on most brands.
When the gas valve is the problem
A gas control valve is the most expensive part on a tank heater, and its price changes the repair math. On a heater under six or seven years old, replacing it in the $350 to $650 range is usually fine. On a heater approaching ten years, that same money is a third of the way to a new one, and the tank has fewer good years left than the valve.
Some valves also fail in a way that matters for safety: the thermostat inside stops reading right and the burner overheats the water. If your hot water is suddenly scalding, or the T&P valve starts discharging, shut the heater off and have the valve checked rather than turning the dial down and hoping.
Gas valve replacement means opening and resealing the gas connection and leak-testing it. That is licensed work in every state, and many cities require a permit for it.
Venting, drafts and carbon monoxide
A pilot that goes out on windy days, or in a closet where the door was just weatherstripped, may be suffering from a draft problem rather than a bad part. Atmospheric gas heaters need a steady supply of combustion air and a flue that draws properly. When the flue backdrafts, exhaust spills out at the draft hood and the burner can starve.
Signs of a venting problem include soot or rust around the draft hood, melted plastic on nearby pipes, condensation on the walls of the heater closet, and a carbon monoxide alarm going off. Treat any of those as urgent. Turn the heater off, open a window and get it inspected before relighting.
Every home with a gas water heater should have a working carbon monoxide alarm on each sleeping floor. They cost less than a pizza and they are required by code in most states.
Power vent and direct vent heaters
Some gas tanks use a fan to push exhaust out through a plastic pipe in the wall. These power vent heaters plug into an outlet and use a hot surface igniter or an electronic pilot. If the power is out, or the outlet's breaker trips, they will not heat at all, even though the gas is on. Check that the unit has power before assuming a gas problem.
Power vent heaters also have a pressure switch that confirms the fan is moving air. A blocked intake or exhaust (bird nests, snow drifts, a clogged condensate trap) makes the switch open, and the heater shows an error instead of lighting. Clearing the termination outside is often all it takes. Fan motor replacement, when needed, typically lands in the $300 to $650 range.
Gas heaters in garages and basements
Heaters in cold garages face an extra challenge in winter: colder incoming water means longer burn cycles, and cold drafts under the garage door push the pilot around. In basements, the issue is usually lint and dust from a nearby dryer or workshop clogging the air intake. If yours lives next to the laundry, ask the plumber to clean the intake screen on every visit.
Heaters in garages also have to sit on a stand that raises the burner at least 18 inches off the floor unless the model is rated as flammable vapor resistant. That rule exists because gasoline vapor is heavier than air and settles low.
Questions people ask
Why does my water heater pilot light keep going out?
Most often a worn thermocouple or thermopile. Other causes are a dirty pilot, a clogged air intake screen, drafts, or a failing gas control valve.
How much does a thermocouple replacement cost?
Usually $150 to $350 installed. Sealed-burner heaters take longer to open up, which pushes the price toward the top of the range.
Is it safe to relight a water heater pilot myself?
Yes, if you follow the label on the heater and there is no gas smell. If you smell gas, leave the house and call the gas company from outside.
What does a blinking light on my gas water heater mean?
It is a status code. A steady blink often means normal operation; a pattern of flashes points to a specific fault. The chart is printed on the heater label.
Should I replace the gas valve or the whole water heater?
If the heater is under about seven years old, replace the valve. If it is near ten years or older, a new heater is usually the better use of the money.