If the pilot goes out as soon as you release the knob, either you need to hold it longer or the thermocouple is weak, dirty or not sitting in the flame. If it stays lit for a while and then goes out, look at a dirty pilot, drafts, low gas supply, or on vent-free units an oxygen depletion sensor doing its job. Thermocouple, thermopile and valve work should go to a fireplace pro.
Key points
- Hold the control knob in for 30 to 60 seconds after the pilot lights so the thermocouple can heat up.
- A healthy pilot flame is blue, steady and wraps the tip of the thermocouple and thermopile.
- A weak, yellow or lifting pilot flame usually means a dirty pilot orifice or hood.
- Thermocouples typically put out about 25 to 30 millivolts. Below that, they get replaced.
- Smell gas? Shut off the gas, leave, and call the gas utility or 911.
How a standing pilot works
On a standing pilot fireplace, a small flame burns all the time next to two sensors. The thermocouple makes a tiny electrical current when heated, about 25 to 30 millivolts, which holds a magnet in the gas valve open. If the pilot goes out, the thermocouple cools, the magnet releases, and the valve shuts off all gas. That is a safety feature, and it is why the pilot will not stay lit when anything in that chain is weak.
Most fireplaces also have a thermopile, a bundle of thermocouples that makes several hundred millivolts. It powers the valve that opens the main burner when you flip the wall switch or press the remote. Some units use one sensor for both jobs.
Vent-free gas logs add an oxygen depletion sensor, or ODS, built into the pilot. It shuts the unit off if oxygen in the room drops.
If your fireplace has no standing pilot and lights on demand, see our guide on gas fireplaces that won't turn on, which covers electronic ignition.
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Safety first
If you smell gas strongly, do not try to light the pilot. Turn off the gas at the key valve or appliance shutoff if you can do it without lingering, do not use switches or phones in the room, get everyone out, and call your gas utility or 911 from outside.
Common causes, in the order techs see them
1. Releasing the knob too soon
After the pilot lights, the thermocouple needs time to heat. Most manuals say to hold the knob in for 30 to 60 seconds. After a new propane tank or a long shutdown, air in the line can take longer to clear.
2. Dirty or worn thermocouple
Soot and oxidation build up on the tip and lower its output. Thermocouples also simply wear out after years of sitting in a flame. One putting out under about 25 millivolts usually needs replacing.
3. Weak or dirty pilot flame
Dust, lint, pet hair and spiders block the tiny pilot orifice or the pilot hood. The flame turns small, yellow, or lifts away from the sensors so they never get hot enough. A good pilot flame is blue with a little yellow at the tips and engulfs the top of the thermocouple and thermopile.
4. Thermocouple out of position or loose
If the sensor gets bumped away from the flame during cleaning or log placement, it will not heat fully. A thermocouple that is loose where it threads into the gas valve can also lose its signal.
5. Drafts and venting issues
Strong wind, a downdraft in the vent, or a door or window nearby can blow the pilot out or push it away from the thermocouple. If it goes out mostly on windy days, the vent termination may need attention.
6. Low gas supply
A low propane tank, a partly closed key valve, or a natural gas problem elsewhere in the house can starve the pilot.
7. ODS trip on vent-free units
If a vent-free unit runs for a while and then shuts down, the ODS may be reacting to low oxygen, or it may be dirty. Crack a window and see if it changes. ODS pilot assemblies are replaced as a unit, not repaired.
8. Failing gas valve
When the thermocouple tests good and the pilot looks right but it still will not hold, the magnet inside the valve may be weak. That is a pro repair.
What you can safely check
- Read the lighting instructions. They are usually on a plate below the firebox or behind the lower louver.
- Confirm the gas is on. Check the key valve or appliance shutoff is fully open and the propane tank has gas.
- Light the pilot and hold the knob in for a full 60 seconds before letting go.
- Look at the pilot flame. Is it blue and strong, and does it wrap the tip of the thermocouple and thermopile? Take a photo for your tech.
- With the gas off and the unit cold, look for spider webs, dust or lint around the pilot. A soft brush or a gentle puff of canned air can clear loose debris. Do not poke wires into the orifice.
- Check for drafts. Close nearby doors and windows and see if the pilot holds.
If the pilot still will not stay lit, stop there. Replacing a thermocouple, thermopile, pilot assembly or valve is gas work and belongs with a trained fireplace tech.
What a pro does and when to call
Call a pro if the pilot will not hold after a full 60-second hold, if the flame is weak or yellow, if it goes out repeatedly, if you see soot on the glass, or any time you smell gas. A fireplace tech will test the thermocouple and thermopile with a millivolt meter, clean the pilot orifice and hood, check gas pressure, inspect the vent, and replace parts as needed. Many techs in our network carry common pilot parts or have dealer accounts, so the fix often happens in one visit. Browse gas fireplace repair pros or find a fireplace tech by ZIP. Annual service catches most of these problems before winter. See our gas fireplace maintenance guide and the fireplace cleaning page. For typical visit pricing, see gas fireplace service costs.
Parts that usually fix it
- Thermocouples, thermopiles, pilot assemblies and valves: see fireplace parts costs.
Pilot parts are specific to the valve and the brand. Heat & Glo, Regency, Napoleon and Majestic use different pilot assemblies, and a part that looks close may not seat correctly in the flame. Find the model and serial number on the rating plate, usually on a chain or tag below the firebox or behind the lower grille, before ordering. Learn more about Heat & Glo fireplaces or Regency fireplaces. If you need help finding a part, ask Pro Grill to find it.
Frequently asked questions
How long should I hold the pilot knob on a gas fireplace?
Most manuals say 30 to 60 seconds after the pilot lights. After a tank change or a long shutdown, it may need a bit longer while air clears from the line.
Can I clean a fireplace thermocouple myself?
You can gently brush loose soot and dust from around the pilot with the unit off and cold. Testing and replacing the thermocouple is gas work, so we recommend a fireplace tech.
Why does my pilot stay lit but the fireplace goes out after a while?
On vent-free units, the oxygen depletion sensor may be tripping. On vented units, it can be a weak thermopile, a dirty pilot, or a draft in the vent. Have a tech test it.
Should I leave my fireplace pilot on all year?
Many owners turn it off in summer to save gas, and some manufacturers recommend leaving it on in damp climates to keep moisture and insects out. Check your manual.
What does a yellow pilot flame mean?
Usually a dirty pilot orifice or hood, or poor air mix. It often leads to soot and a pilot that will not hold. Have it cleaned and adjusted.
