Pilot Won't Stay Lit in October: The Thermocouple, the Thermopile, and the $250 Fix

September 4, 2026

Every October in Fort Collins, the first cold week produces the same phone message a few hundred times: “The pilot lights but goes out when I let go of the knob,” or its cousin, “The pilot is lit but the fireplace won’t turn on.” Both are the same family of fault, both are diagnosed in about two minutes with a millivolt meter, and both usually cost $200 to $400 to fix. This post explains what is happening behind the bottom louver, what you can safely try yourself, and when to stop trying.

If you smell gas at any point, stop reading, leave the house, and call Xcel Energy or 911 from outside.

Why October

The Colorado Climate Center’s Fort Collins station on the CSU campus has been keeping records since 1889, and its 1991-2020 normals put January at a 45.0 degree high and an 18.3 degree low, with 6.7 inches of snow in January alone and 26.1 inches across October through January. That is a real heating season, and most Fort Collins fireplaces work for it from October through March. The other six months they sit cold, and that idle stretch is what kills pilots:

• Dust. Six months of house dust settles into the pilot hood and the burner ports. A pilot flame that should be a sharp blue cone wrapping two sensors becomes a soft yellow wisp that heats neither.
• Spiders. They like the smell of the odorant in natural gas and build in the pilot tube and the burner venturi. A web across the pilot orifice starves the flame.
• Oxidation. The thermocouple tip, which lives in the flame and is meant to be hot, corrodes when it sits cold and slightly damp for half a year. Its output drops.
• Draft. The first cold night pulls a stiff draft down the vent, and a marginal pilot flame that held in the still air of September leans away from the sensors in October.

None of this happens in a summer-off furnace to the same degree because the furnace runs in a closed cabinet. A fireplace pilot lives in the open firebox.

What the pilot actually does

On a standing-pilot fireplace (the kind where a small flame burns all the time, which covers most of the direct-vent units in the late-20th-century subdivisions on Fort Collins’ south side and nearly all the gas log sets in Old Town’s masonry fireplaces) the pilot flame does two jobs through two sensors that sit in it.

The thermocouple is a single junction of two metals. Heated by the pilot flame, it generates a few millivolts, and that tiny current holds an electromagnet in the gas valve that keeps the pilot gas flowing. Let the flame go out, or let the thermocouple get too weak, and the magnet releases, the pilot valve snaps shut, and the gas stops. That is the safety system, and it is why a pilot that will not stay lit is a thermocouple story.

The thermopile is a stack of thermocouples in one probe. It generates roughly 300 to 750 millivolts open circuit, enough to run the main burner valve when the wall switch, thermostat, or remote receiver closes the circuit. No batteries, no house power. When the thermopile sags below about 300 millivolts under load, the switch does nothing, and that is why a pilot that is lit but will not fire the burner is a thermopile story.

Two symptoms, two sensors, one flame. The pilot and ignition repair page covers both in more depth.

What you can safely try

1. Read the lighting label on the valve or inside the louver. Every unit has one, and the sequence matters.
2. Turn the knob to Pilot, push it in, and hold it. Light with the piezo button or a long lighter through the access hole. Once the pilot catches, keep holding for a full 60 seconds. Most “won’t stay lit” calls in the first week are people releasing at 10 seconds, before the thermocouple is hot enough to hold the magnet.
3. Release. If it holds, turn to On and try the wall switch. If it dies, wait five minutes for any unburned gas to clear, and try once more with a full minute of holding.
4. If it still dies, stop. Repeated attempts put gas into the firebox. A thermocouple that cannot hold after two full-minute tries is not going to hold on the tenth.
5. If the pilot holds but the switch does nothing: fresh batteries in the remote and the receiver, and check that the receiver’s slide switch is not on Off. Then stop; the rest is a meter test.

Do not spray anything into the pilot. Do not bend the thermocouple toward the flame with pliers. Do not remove the glass on a direct-vent unit to get a better look while it is running.

How the repair visit goes

The tech pulls the bottom louver, puts a combustible gas detector on the valve and fittings, and then does the two-minute test that decides everything:

• Pilot flame check. Is it a sharp blue cone wrapping both sensors, or a lazy yellow wisp? A dirty pilot gets cleaned first, because half of these calls end right there.
• Thermocouple output. Measured at the valve terminal with the pilot lit. Low with a good flame: thermocouple. Fine with a good flame but the pilot still drops: valve magnet.
• Thermopile output. Open circuit (should be roughly 300 to 750 millivolts) and then under load with the switch closed. Low: thermopile. Fine but the burner will not fire: jump the valve terminals; if it lights, the switch or wiring is the fault, and you get a $150 to $400 switch instead of a thermopile you did not need.
• Altitude and pressure. Fort Collins sits at 4,997 feet. The National Fuel Gas Code calls for derating gas appliances installed above 2,000 feet by about 4 percent per 1,000 feet, and a pilot on a unit that was never derated is often marginal on its own. Manifold pressure gets checked against the manufacturer’s figure for the elevation.

Then the part goes in. On most units the thermocouple and thermopile screw into the pilot bracket and the valve, and the whole job is under an hour. When the pilot hood, tube, and both sensors come as one assembly, or the pilot cannot be cleaned back to a proper flame, it is a pilot assembly at $300 to $600.

What it costs

The diagnostic fee rolls into the repair. The pricing page has the full table. Note the last row: a valve is condemned only after the pilot, the thermocouple, the thermopile, and the switch all pass. A valve quoted without a millivolt reading is a guess, and an expensive one.

Electronic units fail differently

If your fireplace has no standing pilot and clicks when you turn it on, it is an intermittent pilot ignition unit, common in the post-2010 builds in Timnath, Wellington, and Windsor. Its October failure is not a thermocouple. It is dead batteries in the backup pack behind the louver (four AA cells, typically), a fouled spark electrode, or a flame-sensing rod that lost the pilot. Batteries first, always. The pilot and ignition page walks through the rest.

How to not have this problem next October

Book annual service in September. It is $175 to $350 per fireplace, it cleans the pilot and burner before the dust has a chance to matter, and it measures the thermopile while the schedule is open. A thermopile reading 280 millivolts in September gets replaced on that visit for the same $200 to $400 it would cost in January, without the wait, because the week after the first hard freeze is the busiest of our year. Two fireplaces in the house, or a neighbor on the same street, gets the per-unit discount.

And if the honest answer to your message is “hold the knob for a full minute and put batteries in the remote,” you will get that answer for free.