The short answer
- Start with the simple things: the gas valve on, the remote and receiver batteries fresh, and the lighting steps on the label followed exactly.
- A pilot that lights but goes out when you let go of the knob usually points to the thermocouple or a dirty pilot, not a gas supply problem.
- On vent-free logs, the oxygen depletion sensor will shut the pilot off by design if it is dirty or the room air is poor.
- If you smell gas, see soot, or have to relight more than once, stop and call. Do not keep trying.
Before anything else: the safe first checks
Find the lighting instructions. They are on a label or card near the controls, often behind the lower grille or on a chain under the logs. Follow them exactly for your model. Then work through the easy causes:
- Is the gas valve on? The key valve on the wall or floor, or the ball valve under the logs, may have been turned off in spring.
- Is the gas on to the house? Check that another gas appliance, like the water heater or range, works.
- Are the batteries fresh? Remotes, wall receivers and some igniters run on batteries, and a weak set is the most common reason a remote system will not respond.
- Is the control knob on Pilot when you press it, and are you holding it in long enough? Many pilots need you to hold the knob down for 30 seconds or more after the pilot lights.
- Has the system been off all summer? Air in the line can take a few tries to clear on the first light of the season.
Pilot lights, then goes out: thermocouple and thermopile
Most gas logs with a standing pilot have a safety device that keeps the gas valve open only while the pilot flame is hot. On many sets it is a thermocouple: a small metal rod that sits in the pilot flame and makes a tiny electric current when heated. If the pilot goes out, the current stops and the valve closes.
Sets with a wall switch or remote often also have a thermopile, a bigger version that makes enough current to open the main burner valve. A weak thermopile shows up as a pilot that stays lit while the logs will not come on from the switch.
So when the pilot lights but dies the moment you release the knob, the usual suspects are a thermocouple that has worn out, a pilot flame that is too small or not touching it, or a dirty pilot. Replacing a thermocouple or thermopile is a repair for a technician, because it means working on the gas valve.
Vent-free logs and the ODS
Vent-free gas logs burn with the flue closed or no flue at all, so they have one more safety device: an oxygen depletion sensor, or ODS, built into the pilot. If the oxygen in the room drops, the pilot flame lifts away from the sensor and the gas shuts off. The manuals for these sets describe it shutting down before oxygen falls below about 18 percent.
The ODS works exactly as designed most of the time, which means a dirty ODS pilot will shut your logs off even when the room air is fine. Dust, pet hair and spider webs in the small pilot opening change the flame. A pilot that goes out after the logs have run a while in a closed room can also be the ODS doing its job. Crack a window and do not keep relighting it.
Dirty pilots and the seasonal restart
Over a summer, dust settles in the pilot assembly and spiders build in the tubes. You can gently vacuum loose dust from around the logs and burner with the gas off and everything cold. Do not take apart the pilot, poke into orifices, or move the logs out of the positions shown in the manual. Log placement affects how the flame burns and how much soot and carbon monoxide it makes.
A yearly service before the cold weather is the simplest way to avoid a no-start night. That means cleaning the pilot and burner, checking the thermocouple or thermopile output, checking the logs for damage and placement, testing for leaks at the connections, and making sure the damper or flue is set right for the type of logs you have.
Signs to stop and call
There is a point where trying again is the wrong move. Stop if you smell gas, if the flame is tall, yellow and sooty, if black soot is building on the logs or the fireplace opening, if the pilot will not stay lit after following the label twice, or if the main burner lights with a whoomp. If the smell of gas is strong, leave the house and call 911 or the gas utility from outside.
Also make sure there is a working carbon monoxide alarm in the room or next to it, especially with vent-free logs.
When to call a pro
Call for service when the pilot will not stay lit after following the label, the logs light but burn yellow or sooty, the remote or switch will not start a lit pilot, you smell gas, or the set has not been serviced in years. A seasonal restart before the first cold snap avoids most of these.