Why the pilot goes out, and what you can safely check

Safety first

When a gas fireplace pilot light keeps going out, a safety system is doing its job: once the flame disappears, the valve shuts off the gas. The risk comes from relighting carelessly. Any gas smell ends the attempt: leave the house without touching switches or using a phone inside, and make the call to the gas utility or 911 from outdoors.

When you relight, follow the lighting instructions on the unit's label exactly. The opening steps on the label are the part to respect: knob to off, a five-minute wait so any gas can clear, then a sniff for gas low near the floor before you continue. Skipping the wait is how an unburned pocket of gas gets lit.

How the pilot keeps the valve open

On a standing-pilot gas fireplace, a thermocouple sits in the pilot flame. Heat at its tip makes a small voltage, measured in millivolts, and that voltage energizes a safety magnet inside the gas valve that holds the pilot gas path open. While you light the pilot, you hold the knob in by hand, which does the magnet's job until the thermocouple warms up. When you let go, the thermocouple has to take over. If it cannot, the magnet releases and the pilot goes out.

Some units also have a thermopile, a stack of thermocouple junctions in the same pilot flame, whose larger output powers the main burner through the wall switch or remote. Units with electronic ignition use a flame rod and a control module instead, and a pilot that lights and drops out on those units is a module or sensing fault rather than a thermocouple one.

Cause 1: letting go of the knob too soon

The simplest cause is timing. The lighting instructions say how long to keep the knob pressed after the pilot lights, and one maker's instructions call for a full minute. Let go early and the thermocouple is not yet hot enough to hold the magnet, so the pilot goes out even though every part is fine.

Try once more, holding for the full time the label gives. If the pilot now stays lit, nothing is wrong. If the pilot drops again despite a full hold, the thermocouple is not getting hot enough or its signal is not reaching the valve, and the causes below apply.

Cause 2: a weak or misplaced pilot flame

The manual shows what the pilot flame should look like. In one maker's drawing of the pilot assembly, three flames come off the pilot hood: one for the thermocouple, one for the thermopile and one aimed at the burner. A flame that is small, yellow or lazy, or that leans away from the sensor, leaves the thermocouple too cool.

The likely culprit is dirt in a very small place. Dust, a spider web or a bit of scale in the pilot orifice or its air opening shrinks the flame. You can look at the flame through the glass. Cleaning the pilot means opening the sealed firebox and working on the gas path, and gas fireplace manuals reserve that for a qualified service person, so this is where the owner's part ends.

Cause 3: a worn thermocouple or thermopile

Thermocouples wear. The tip sits in a flame for years, oxidizes and gradually makes less voltage, until one day it cannot hold the valve. A technician tests it instead of guessing: the lead is read with a millivolt meter while the pilot burns and the knob is held in, and the reading is compared with the figure in the maker's troubleshooting chart.

A low reading with a good flame means the thermocouple, or on some units the whole pilot assembly, is replaced with the part listed for the model. If the reading is good and the pilot still drops, the valve is next in line.

Cause 4: a loose connection at the valve

The thermocouple's lead screws into the gas valve, and its voltage is small enough that a loose or dirty connection can lose it. Maker troubleshooting charts include a step to make sure the thermocouple connection at the valve is fully inserted and tight. The valve sits in the control compartment and is part of the gas path, so this check belongs to the service visit, and a technician makes it early, before any part is replaced.

Cause 5: drafts, vents and spill switches

A pilot can also be blown out or starved. On a natural vent, or B-vent, unit, which draws its air from the room, a downdraft in the vent or a strong draft across the room can push the pilot flame off the sensor. Gas fireplace manuals say not to install units in windy or drafty areas for that reason.

Some B-vent units also have a spill switch, which closes the valve if exhaust rolls back into the room. When it trips, the pilot drops out and, on some units, will not stay lit until the switch is reset, and the trip itself is a sign the vent needs attention.

On a sealed direct-vent unit, room drafts do not reach the pilot, but the vent does. A termination blocked by a nest, leaves or a shrub, or a leak inside the vent pipe that lets exhaust into the intake, can make the pilot and burner go out while the unit runs. If the pilot goes out on windy days, or only after the burner has been running a while, the vent is the place to look.

Cause 6: the gas supply

A pilot needs steady gas pressure. If a propane tank is running low, or the supply to the house is weak, the pilot can shrink or drop when another appliance starts. Check whether other gas appliances work normally and, on propane, look at the tank gauge. If the pilot goes out at the moment the main burner comes on, low pressure is a strong suspect. Supply pressure is measured at the valve's inlet test port during a service visit, and a supply problem is for the gas utility, a gas contractor or, on propane, the supplier.

Vent-free units and the oxygen depletion sensor

Vent-free gas fireplaces and log sets use a different pilot, an oxygen depletion sensor, or ODS. The fuel gas code requires unvented room heaters to shut off when oxygen in the room drops to the level the maker specifies, within a limit the code sets, and the ODS pilot is how they do it. A dirty ODS pilot can trip early, and so can a room with too little fresh air. If a vent-free unit's pilot keeps going out, read it as a message about the room air as well as the pilot, and have it serviced; ventless gas fireplace repair has a page of its own.

Owner checks, and the point to hand over

The owner-level checks are short: no gas smell, the lighting instructions followed exactly with the knob held for the full time, a look at the pilot flame through the glass, other gas appliances working, the propane gauge if you have one, and the vent termination outside clear of leaves, nests and plants. If the fireplace will not turn on at all, the switch, remote and supply checks are in the guide on what to do when a gas fireplace won't turn on.

Stop and call for service when the pilot goes out after a full hold, goes out repeatedly, goes out when the burner starts or after it has run a while, when a spill switch trips, when you see soot or a yellow, lazy flame, or when water has reached the valve or controls. Each of those needs the unit opened and tested.