Furnace pilot light won't stay lit: what's usually behind it
A pilot flame that lights and then dies again a few seconds later is a specific, recognizable pattern. And on most furnaces old enough to have a standing pilot, one small safety part is the most common cause.
First, does your furnace actually have a standing pilot?
Furnaces built roughly in the last two decades mostly use electronic ignition instead. Either a hot surface igniter (a small glowing element) or an intermittent pilot that only lights right before each heating cycle and then shuts off, rather than burning continuously.
Look through the furnace's viewing port. If you never see a small, continuously burning flame, including when the furnace isn't calling for heat, it likely doesn't use a standing pilot at all, and this pattern doesn't apply. A furnace with no flame ever visible is a different troubleshooting question, closer to an ignition-system fault than a pilot problem.
The thermocouple is the most common cause
On furnaces that do have a standing pilot, a thermocouple is a small safety sensor positioned in the pilot flame. Its entire job is to confirm the pilot is actually lit by sensing its heat. And to signal the gas valve to stay open only while that heat is present.
A thermocouple can get dirty, drift out of position within the flame, or simply wear out. Then it stops sensing enough heat even with the pilot burning normally. The valve does exactly what it's designed to do, which is shut off gas to a flame it can no longer confirm. That is why the pattern looks like "it lights, then goes out again a few seconds later". The safety system is working correctly, even though the pilot itself may be fine.
| Possible cause | What's happening |
|---|---|
| Worn, dirty, or misaligned thermocouple | The sensor can't confirm the flame's heat, so the safety valve shuts gas off as designed |
| Draft near the pilot | Air movement from a nearby vent, door, or gap can blow out a small flame repeatedly |
| Clogged pilot orifice | Dust or debris at the tiny gas opening produces a weak, unstable flame that's easily lost |
| Weak or fluctuating gas supply, or a failing gas valve | Not enough gas reaching the pilot reliably to sustain a flame |
Other causes worth knowing about
A draft reaching the pilot (from a nearby vent, an unsealed gap. Or even air movement inside the furnace cabinet) can blow out a small flame the same way it would blow out a candle.
A clogged pilot orifice, the tiny opening the gas passes through, can leave the flame weak and unstable enough that it doesn't stay lit even with a working thermocouple. Less often, the cause sits further upstream: fluctuating gas supply pressure or a gas control valve that's starting to fail can also keep a pilot from holding.
If there's ever a smell of gas, stop
None of the causes above should be diagnosed or worked on if you smell gas at any point, before or during an attempt to relight anything. Leave the house immediately and call your gas utility or 911 from outside: never keep trying to light a pilot. Or investigate further, once there's a gas odor present.
What a homeowner can reasonably check
If there's no gas smell, check the unit itself. Most furnaces with a standing pilot have a relighting label fixed to the cabinet, because the exact sequence (which knob, how long to hold it, in what order) varies by model and manufacturer.
Following that printed label exactly, once, is reasonable. If the pilot won't stay lit after following it once, the pattern points at a worn part like the thermocouple rather than user error. And repeating the attempt over and over isn't going to change that.
What not to do
- Do not keep attempting to relight the pilot repeatedly if it won't hold after one attempt following the label's instructions: a worn thermocouple or clogged orifice won't fix itself by trying again.
- Do not try to clean, bend, or reposition the thermocouple or the pilot assembly yourself. It's a small, specific safety part, and a licensed HVAC technician tests and replaces it correctly.
- Do not attempt to light anything, or keep troubleshooting, if you smell gas at any point: leave and call instead.
When it's a call to a technician
A thermocouple is an inexpensive, common replacement part, and a technician can typically confirm it and swap it in a single visit. Suppose the pilot lights and holds fine but the main burners still don't fire. Or the furnace has an intermittent or hot-surface ignition system rather than a standing pilot at all. Either way that's a different fault inside the ignition or control system: also technician territory rather than something to keep testing yourself.
Frequently asked questions
How do I know if my furnace even has a pilot light?
Look through the furnace's viewing port or access panel for a small window. If you see a continuous, small flame burning at all times (including when the furnace isn't actively heating) it has a standing pilot. If you never see a flame there, or only briefly right as the furnace starts a cycle, it more likely uses electronic ignition instead, which is a different system entirely.
What does a thermocouple actually do?
It's a small safety sensor sitting in the pilot flame that confirms the pilot is genuinely lit by sensing its heat, and keeps the gas valve open only while that heat is present. If it's worn, dirty, or out of position, it stops sensing enough heat even with a normal pilot flame, and the valve shuts off gas as designed: which looks like the pilot "won't stay lit."
Is it safe to keep relighting the pilot myself if it doesn't hold?
Following the printed relighting label on the furnace once, with no gas smell present, is reasonable. Repeating the attempt over and over when it doesn't hold isn't going to fix a worn thermocouple or a clogged orifice. At that point it's a part for a licensed technician to test and replace, not something that improves with more tries.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Furnaces
- CDC. Carbon Monoxide Poisoning Basics