Why won't my water heater pilot light stay lit?

The thermocouple is the part that decides whether your pilot stays lit. It is a thin metal probe that sits right in the pilot flame. When the tip gets hot, it produces a tiny electric signal. That signal tells the gas control valve it is safe to keep gas flowing to the pilot. No heat at the tip means no signal, and the valve shuts the gas off as a safety step.

This is why the timing matters. You hold the pilot knob down, light the flame, and it burns fine. Then you release the knob and the pilot dies within seconds. That pattern almost always means the thermocouple is bad, dirty, or out of position. The valve never got the "keep going" signal, so it closed.

A few things knock a thermocouple out of service. Soot or scale builds up on the tip and blocks the heat. The probe gets bent out of the flame, so it never warms enough. Or the part simply fails with age after years of heat cycles. A new thermocouple is an inexpensive part, often under $20, and is one of the most common water heater repairs.

Position is easy to check by eye. The pilot flame should be a steady blue cone that wraps the top portion of the thermocouple tip. A good rule is that the flame should cover about 3/8 inch of the tip. If the flame is small, yellow, or lifting away from the probe, the thermocouple cannot stay hot enough to hold the valve open.

Several other faults mimic a bad thermocouple, so it helps to rule them out. A dirty pilot orifice is high on the list. The orifice is the tiny opening that feeds gas to the pilot. Dust, spider webs, and rust flakes clog it over time. A starved pilot burns small and weak, drifts off the thermocouple, and dies. Cleaning or replacing the orifice restores a full flame.

Newer tanks add another suspect. Most gas units built since about 2003 are FVIR models, which stands for flammable vapor ignition resistant. These have a sealed combustion chamber and an air-intake screen, also called a flame arrestor, on the bottom. The screen pulls in the air the pilot and burner need to breathe. In a dusty garage, or near a lint-heavy laundry area, that screen clogs and starves the pilot of air. The flame shrinks and goes out even though the thermocouple is fine. Many FVIR units have a sight window so you can watch the flame and a reset feature for a tripped safety device.

The gas control valve itself can fail. This is the box on the front of the tank with the temperature dial and the pilot knob. Inside is the magnet that the thermocouple signal holds open. When that magnet or the valve electronics wear out, the valve will not hold the pilot even with a perfect thermocouple. A failed gas valve is a bigger repair, and on an older tank it often tips the math toward replacement.

A few simple things matter too. Low gas pressure or a partly closed supply line gives the pilot too little fuel. A strong draft from a nearby door or vent can blow a marginal flame out. And on a unit with old age behind it, more than one of these problems can stack up at once.

Some gas water heaters use a thermopile instead of a single thermocouple. A thermopile is a bundle of several thermocouples wired together. It does the same job of sensing the pilot flame, but it makes far more voltage, enough to run the whole gas valve and its controls without any outside power. These are called millivolt systems, and you find them on many modern tanks with electronic gas valves and a push-button igniter.

The symptom looks the same. The pilot lights but will not stay lit once you let go. The fix is similar in spirit but not identical. A weak thermopile cannot produce enough millivolts to satisfy the valve, so the valve closes. A technician confirms this with a meter that reads the thermopile output. A healthy thermopile usually reads in the hundreds of millivolts, and a reading well below the valve's listed minimum points to a worn part.

The takeaway is to identify which system you have before buying parts. A standard standing-pilot tank uses a thermocouple. A push-button or electronic-ignition tank likely uses a thermopile. Your owner's manual names the part and lists the correct replacement, which is why matching the manufacturer's manual to your exact model matters.

Start by reading the lighting label on the tank and the steps in your manual, because details vary by brand and model. The general sequence is the same across most standing-pilot units, but always follow the printed instructions on your own heater.

1. 1Smell for gas first. If you smell rotten eggs or sulfur anywhere near the heater, stop. Do not touch any switch, knob, or flame. Leave the home and call for help. See our page on a house that smells like gas or rotten eggs for the full safety steps.
2. 2Turn the gas control knob to OFF and wait at least five minutes for any gas to clear. This wait is required, not optional.
3. 3Turn the dial to the lowest temperature, then set the knob to PILOT.
4. 4Press and hold the pilot knob down. Light the pilot with the igniter button or a long match at the pilot tube.
5. 5Keep holding the knob for about 30 seconds to one minute after the flame catches. This lets the thermocouple heat up.
6. 6Release the knob slowly. If the pilot stays lit, turn the knob to ON and set your temperature. The U.S. Department of Energy recommends 120 degrees Fahrenheit for safety and energy savings.

If the pilot dies again after two careful tries, stop. Repeated relighting will not fix a worn part, and it wastes gas. The problem is mechanical, and it needs a part or a closer look.

Know when to stop and call a pro. A gas smell means stop right away and treat it as a leak. Southwest Gas states clearly: "If you smell natural gas, hear the hissing sound of gas escaping or see other signs of a leak, immediately evacuate the area and then call 911 and Southwest Gas." Call from a safe place away from the home. Also call a licensed plumber if the gas valve looks corroded, if water is leaking near the burner, or if the unit is old and the repairs are stacking up.

Sometimes the smarter move is a new tank, not another part. Age is the first thing to weigh. A typical storage water heater lasts about 8 to 12 years. If a unit near or past that age needs a gas valve, the repair cost climbs toward a good share of a replacement, and the rest of the tank is aging too. Hard water in the Phoenix area shortens tank life and adds sediment that strains every part.

Water heating is worth getting right because it is the second-largest energy user in a typical home, behind only heating and cooling. A failing tank that short-cycles or runs a dirty burner wastes fuel every day. A clean, correctly sized replacement runs more efficiently and ends the cycle of small repairs.

Replace, rather than repair, when you see corrosion on the gas valve body, rust-colored water, water pooling at the base of the tank, or a pattern of pilot and burner failures over a short span. A single dirty thermocouple on a five-year-old tank is a quick fix. The same fault on a twelve-year-old tank with a leak and a tired valve is a sign the tank has reached the end. When the numbers and the age line up against the repair, a new unit is the better spend.

If you are in metro Phoenix and your pilot will not stay lit, our team can diagnose the thermocouple, orifice, air screen, or gas valve and tell you honestly whether a repair or a replacement makes more sense for your tank.