Water Heater Pilot Light Won't Stay Lit: Troubleshooting
You get in the shower and the water runs cold. Down in the basement, the little window on the water heater is dark. You follow the instructions printed on the tank, hold the knob down, watch the pilot catch, let go, and thirty seconds later it dies again. Second try, same thing. Third try, same thing.
That repeating pattern tells you something useful. A pilot that lights and then goes out is behaving differently from a pilot that won't light at all, and the difference points at a specific part.
Before anything else: if you smell gas at any point, stop. Don't try to relight, don't flip switches, don't hunt for the source. Leave the house and call your gas utility from outside, or call 911. Most gas utilities run a 24 hour emergency line and will send someone out for a suspected leak. That call is never an overreaction.
What the pilot light is actually doing
A standing-pilot gas water heater has a small flame burning all the time. Its job is to light the main burner when the thermostat calls for heat, and to prove to the gas valve that a flame exists.
That second job is the safety system. Sitting in the pilot flame is a thermocouple, a thin metal probe that produces a tiny electrical current when it gets hot. That current, measured in millivolts, holds open an electromagnet inside the gas control valve. Flame goes out, probe cools, current stops, and the valve slams shut. That's what keeps raw gas from filling your basement.
So when the pilot lights while you're holding the knob and dies the moment you release it, the flame was fine. The problem is that the valve never got told the flame was there. Some newer heaters use a thermopile instead, which is a bundle of thermocouples generating more voltage, but the logic is the same.
The most common culprit: the thermocouple
Thermocouples are cheap, they sit directly in a flame for years, and they wear out. On a heater that's been running fine and suddenly won't hold a pilot, this is where to start.
Look at the pilot flame while it's lit. You want a steady blue flame with maybe a small yellow tip, and it should wrap around the top half inch or so of the thermocouple tip. Two things go wrong here:
- The flame is weak, small, or lazy and yellow. The thermocouple isn't getting hot enough to produce current. Usually the pilot orifice is partly clogged with dust, lint, or spider webbing. Spiders love gas orifices, and a single web can choke a pilot down to a flicker.
- The flame looks healthy but the pilot still drops out. The probe itself is probably done, or the connection is bad.
The connection is worth checking because it costs nothing. The thermocouple lead threads into the gas control valve as a small nut. If it's loose or the contact surface is corroded, the signal doesn't get through. Snug is enough. Overtightening that fitting is a common way to make things worse.
Replacing a thermocouple is a genuinely common homeowner repair. The part runs a few dollars at any hardware store, and it's a matter of pulling the burner assembly, swapping the probe, and reinstalling. It's also a repair where the failure mode involves gas, so if you're not confident pulling and reseating a burner assembly cleanly, that's a fine place to hand it off.
Check the air supply before you blame parts
Water heaters built in the last two decades have a flammable vapor ignition resistant design, which means a sealed combustion chamber that draws air through a screen or filter around the base of the tank. That screen collects dust, pet hair, and lint. If you have your washer and dryer in the same room as the heater, it collects a lot of lint.
Starve the burner of air and the pilot suffocates. It'll light, run briefly, and go out, which looks exactly like a failing thermocouple. Look at the bottom perimeter of the tank. If you see a felted gray mat where there should be an open screen, that's your answer. Vacuum it clean with a brush attachment and try again.
While you're down there, check whether anything is stored against the heater. Paint cans, gasoline, solvents, and aerosols do not belong in the same room as an open pilot flame. Move them out regardless of what you find.
Hear it handled live. Answara's own AI receptionist answers our main line. Call (857) ANSWARA and ask it whatever a customer would ask you, or see plans and get set up.
Drafts and venting
The vent pipe on top of a gas water heater carries combustion gases, including carbon monoxide, out of the house. When that vent doesn't draw properly, exhaust spills back into the room and can blow out the pilot.
A few things cause it:
- A bird nest, wasp nest, or debris in the flue or the exterior vent cap
- A disconnected or rusted-through vent pipe section, or one that's sloped downward instead of rising toward the chimney
- Negative pressure in the house, most often from a powerful kitchen or bath exhaust fan, or a dryer, pulling harder than the vent can draw
You can check the easy version yourself. With the heater running, hold a lit match near the draft hood at the top of the tank. The flame should get pulled toward the vent. If the smoke pushes back out at you, the heater is backdrafting.
Stop there and call someone. Backdrafting is a carbon monoxide problem before it's a pilot light problem. If your CO alarm has gone off, or anyone in the house has had unexplained headaches, nausea, or drowsiness that improves when they leave, get everyone outside and call the fire department or the gas utility first.
When the gas valve is the problem
If the thermocouple is new, the pilot flame is strong and blue, the air intake is clear, and the pilot still drops out, suspicion moves to the gas control valve itself. The electromagnet inside can weaken with age and stop holding.
There's a second component that gets missed. Many heaters have a thermal switch mounted on the combustion chamber, a one-time cutoff that trips if the chamber overheats. Once it trips, the pilot will not stay lit no matter what you do, by design. That switch tripped for a reason: a blocked air intake, a serious venting fault, or flammable vapors that got into the chamber. Resetting or replacing it without finding the cause is how people get hurt.
Gas control valves are not a homeowner part. Replacing one means disconnecting the gas supply line and reassembling a gas-tight joint. Call a licensed plumber.
Call a plumber when you see these
Some of this is a Saturday morning with a vacuum and a five dollar part. Some of it is not. Stop and call a professional if:
- You smell gas at any point, at which point you leave first and call from outside
- The pilot relights but the heater goes out again within a day or two
- There's soot, scorching, or melted plastic around the burner or draft hood
- There's standing water in the burner compartment or under the tank
- The thermal switch has tripped
- The unit backdrafts when you test the draft hood
- The heater is past roughly twelve years old and the tank is rusting
That last one matters more than people expect. A twelve year old tank with a failing valve is often worth replacing rather than repairing, and a plumber can tell you which side of that line you're on. If you want more background before you call, our plumbing resources cover what to expect on a service visit.
The part nobody talks about: actually reaching a plumber
Water heaters fail at inconvenient hours. Cold showers get discovered at six in the morning, and pilot lights tend to give up on the first genuinely cold night of the season, when every plumber in the county is already booked solid.
So the homeowner calls. It rings out, or lands in a full voicemail box, and they hang up and dial the next name on the search results page. Very few people leave a message when they have no hot water and a family waking up. Across the service trades, missed calls are one of the quietest ways work walks out the door, and the shop that lost the job usually never learns it existed.
For a plumbing business, that's the gap worth closing. An AI voice receptionist like Answara answers the six a.m. no-hot-water call, asks whether there's a gas smell, flags the emergency, and books the ones that can wait for a normal slot. The homeowner gets an answer instead of a dial tone, which is most of what they wanted at six in the morning anyway.