Gas Fireplace Pilot Won't Stay Lit
The pilot lights while you hold the knob, then dies the moment you let go. That single symptom points at a very short list of parts, and one of them fails far more often than the rest.
Why it dies the second you let go
Holding the knob down feeds gas to the pilot manually. Letting go hands that job to a safety device — a thermocouple on older units, a thermopile on newer ones. It sits in the pilot flame, the heat generates a tiny electrical current, and that current is what tells the gas valve it is safe to stay open.
When the flame goes out the instant you release, the valve is not seeing enough current. Either the sensor has worn out, or the flame is not heating it properly.
The four things that actually cause it
1. A worn thermocouple or thermopile. These wear out. A healthy thermopile reads around 350-750 millivolts; below roughly 300 it can no longer hold the valve open. This is the most common cause by a wide margin, and it is an inexpensive part.
2. A dirty or partly blocked pilot orifice. Dust, spider webbing and combustion debris narrow the opening. The flame gets small, lazy and yellow instead of a crisp blue cone, and it no longer wraps the sensor tip.
3. The flame is not hitting the sensor. The pilot flame should envelop the top half-inch of the sensor. A bent bracket, a knocked pilot hood or a log moved out of position can pull the flame off target.
4. A failing gas valve. Least common, most expensive. We rule out the other three first, every time — replacing a valve when a $20 sensor was the problem is the kind of bill we will not hand you.
What you can safely check yourself
If you smell gas at any point, stop. Do not use switches or your phone indoors. Leave the house and call your gas utility from outside. That is not a formality — it is the one instruction here that matters more than the rest.
Otherwise: look at the pilot flame while you hold the knob. A steady blue cone wrapping the sensor is healthy. A small, lazy, yellow or dancing flame is not. Check the log directly above the pilot has not shifted. Confirm the gas is actually on at the shutoff. If the unit has been sitting all summer, air in the line can cause a few failed attempts before it settles — a second or third try is reasonable, a tenth is not.
Do not clean the pilot assembly with compressed air or a wire. It is easy to bend the electrode or crack the ceramic, and you turn a part swap into an assembly replacement.
What a repair visit actually looks like
We put a meter on the thermopile and read the actual millivolt output under load — that number tells us in about a minute whether the sensor is the problem or whether we are looking at the valve. We check the pilot flame shape and the orifice, verify the log placement against the manufacturer's diagram, and confirm gas pressure at the valve.
You get told what is wrong and what it costs before anything is replaced. If it is the thermopile, most of these are done on the same visit.
Do not just keep relighting it
A pilot that needs three attempts today will need ten next month, and it always fails on the coldest night. More importantly, a pilot that keeps dropping out is a safety device doing its job — it is telling you something upstream is not right. Repeatedly overriding it by holding the knob longer is working around the one part designed to protect you.
FAQs
How much does a thermocouple or thermopile replacement cost?
The part itself is inexpensive. What varies is access - some fireboxes give it up in minutes, others need the logs and burner pulled. We give you the full price in writing before we start, so there is no surprise at the end.
Can I replace the thermocouple myself?
It is sold as a DIY part, but the pilot assembly is easy to damage and the replacement has to sit at exactly the right depth in the flame to read correctly. If it is set wrong you get the same symptom back and no way to tell whether the new part is faulty or just misplaced.
Why did it start right after summer?
Months of no use let dust and webbing settle in the pilot orifice, and the sensor sits idle. This is the single most common time of year for this fault, which is why a pre-season service catches most of them before the first cold night.
The pilot stays lit but the main burner will not come on.
That is a different fault - it points at the thermopile, wall switch, remote receiver or valve rather than the pilot safety circuit. The pilot proves gas and ignition are fine; the problem is in what tells the valve to open the main burner.