Quick answer
A gas fireplace pilot light that keeps going out usually has a worn thermocouple or thermopile that no longer holds the gas valve open, or a dirty pilot hood and orifice giving a weak, yellow, lifting flame. If it only drops out on windy days, suspect the vent termination or a leaking glass gasket. If it drops when the furnace fires, suspect low gas pressure.
When a gas fireplace pilot light keeps going out, the timing of the dropout is the best clue you have. A pilot that dies the moment you release the knob, one that dies an hour into a fire, one that dies on gusty nights and one that dies when the furnace starts are four different faults. Note which pattern you have before you relight it. A fireplace that will not light at all is a separate question, answered in why a gas fireplace won't light.
Match the dropout pattern to the cause
- Goes out as soon as you release the pilot knob: the thermocouple, its connection at the valve, or the valve's safety magnet.
- Stays lit alone, goes out when the main burner comes on: a weak or misdirected pilot flame, a leaking glass gasket, or gas pressure sagging under load.
- Goes out on windy days or nights: the vent termination outside.
- Goes out when the furnace, water heater or range is running: low gas pressure on a shared line.
- Out every few days with no pattern: a dirty pilot assembly or a thermocouple near the end of its life.
A failing thermocouple or thermopile
The thermocouple is the pilot's safety device, and it is the first part to suspect on a pilot that will not hold. Its tip sits in the flame and generates a few hundredths of a volt, enough to hold a small electromagnet in the gas valve open. When the flame goes out the tip cools, the magnet releases and the pilot gas shuts off. After years in a flame the tip erodes and its output falls until a slight flicker is enough to drop the valve.
The thermopile is the larger sensor beside it that powers the main burner valve, and some fireplaces rely on it alone for both jobs. Both are checked with a millivolt meter, not by eye. A loose thermocouple nut at the valve produces the same symptom as a dead thermocouple, which is why a technician tests before replacing.
A dirty pilot hood or orifice: weak, yellow and lifting
A healthy pilot is a sharp blue flame that wraps the top centimetre or so of the thermocouple and pushes firmly at the thermopile. A pilot starved by dust, lint or a spider web in the orifice turns soft and yellow, and lifts or wavers off the hood so it only brushes the sensors. They cool, and the valve closes. This is the usual cause on a fireplace that sat unused all summer.
Cleaning the orifice and hood is a strip-down of the pilot assembly with the gas disconnected, followed by a leak test. That is gas work under CSA B149.1 and needs a TSSA licence. Poking the orifice with a pin or wire enlarges it and makes the flame worse.
Wind at the vent termination, and a glass gasket that leaks
A direct-vent fireplace is a sealed box: it draws combustion air in and pushes exhaust out through one two-pipe termination on the outside wall or roof. Anything that upsets that balance moves the pilot flame. A termination cap that is damaged, missing, clogged by a nest or set in a corner where wind swirls lets gusts push straight down the pipe. A house with a wall facing Lake Ontario or a lot on the Niagara Escarpment brow is more exposed than a sheltered subdivision street, so check the cap first there.
The glass front is part of the seal. If the glass was removed for cleaning and not latched evenly, or its rope gasket has flattened or frayed, room air leaks into the firebox, the flame pattern shifts and the pilot gets pulled off the thermocouple. A leaking gasket also lets combustion gases into the room, so it is a repair to make promptly and not a nuisance to live with.
Low gas pressure when the furnace and other appliances fire
If the pilot drops when the furnace starts, the gas line is not delivering enough for everything at once. Natural gas appliances are built for an inlet pressure of about 7 inches of water column, a very low pressure that a long or undersized branch line can pull down when a furnace, water heater and range fire together. Houses that have gained a range, a barbecue line, a pool heater or a tankless water heater since the piping went in are the usual candidates.
A technician connects a manometer at the fireplace valve and watches the reading while every other appliance runs. If it sags, the fix is a gas line upgrade or a regulator correction, not a fireplace part. Pressure problems at the meter itself belong to Enbridge Gas.
How many times to relight, and when to stop
Relight the pilot once, following the label and waiting five minutes for gas to clear. If it goes out again inside a day or two, stop relighting and book service: repeated relights do not fix a weak thermocouple, and each one releases unburnt gas into the firebox. Whether a pilot going out is dangerous explains what the safety valve does and does not protect against.
- Call now: you smell gas with the pilot out. The safety valve should have closed, and it has not.
- Call now: soot on the glass, a carbon monoxide alarm, or a stale smell and heavy condensation in the room while the fireplace runs.
- Can wait for a booked visit: a pilot that drops out occasionally with no smell, no soot and a blue flame.
A pilot repair starts with a $90–$150 diagnostic visit that is folded into the repair, and the part and labour are quoted in writing once the cause is confirmed. If the fireplace is old enough that its pilot assembly is discontinued, we will say so and price a replacement instead; see gas fireplace installation. Reach ZK Mechanical at (647) 801-1252 or through the contact page.