Why does my furnace pilot light keep going out?
Updated August 2026
On a furnace with a standing pilot, a flame that lights but will not stay lit is a dirty or failing thermocouple in the large majority of cases. The thermocouple is a metal probe that sits in the pilot flame and generates a tiny voltage to hold the gas valve open; when it gets sooty or wears out, it can no longer prove the flame, so the valve closes for safety and the pilot dies. It is a ten-dollar part and the first thing to check.
Safety first: this is a gas appliance, so respect a few hard rules. If you smell gas at any point, especially the rotten-egg odorant, do not try to relight anything: leave the house and call your gas utility or 911 from outside. Never hold the gas control down and relight the pilot over and over, because each failed attempt lets unburned gas collect in the burner box. Keep a working carbon-monoxide detector on every level, and if the pilot keeps failing on a furnace older than 15–20 years, have a tech confirm the heat exchanger is not cracked before you keep using it.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| Dirty or failed thermocouple | Most common | The probe cannot sense the flame and closes the gas valve. Gently clean the soot off the tip with fine abrasive, or replace the thermocouple. | $10 part, $20–150 installed |
| Weak pilot flame or clogged orifice | Common | A dust-clogged pilot orifice gives a small yellow flame that barely touches the probe. A tech clears the orifice so the flame runs blue and strong. | $75–200 |
| Drafts blowing the pilot out | Common | A cracked flue, a strong downdraft, or a nearby exhaust vent can gust the pilot out. Seal the draft source or correct the venting. | $50–300 |
| Misaligned thermocouple | Occasional | If the probe tip sits outside the flame, it never gets hot enough. Bend it back so the flame wraps the tip. | Free |
| Failing gas valve | Rare | A worn magnetic valve will not stay open even with a good thermocouple signal. The valve is replaced. | $150–400 |
Check these first
- Follow the lighting instructions on the furnace label exactly: hold the gas knob to PILOT, light it, and keep holding for a full 30 to 60 seconds before releasing.
- Watch the pilot flame color and size. It should be a strong blue flame that clearly envelops the tip of the thermocouple probe.
- If the flame is small and yellow, the pilot orifice is likely clogged and needs a tech to clean it.
- Check for obvious drafts near the burner from a nearby door, vent, or cracked flue pipe.
- Look at the thermocouple tip. If it is coated with black soot, gently clean it with fine sandpaper or an emery cloth and retry.
Prevent it next time
A pilot that keeps dropping out is usually a furnace that missed its checkup. Book a pre-winter furnace test and a burner and pilot cleaning, where a tech clears the pilot orifice, checks the thermocouple output, and inspects the flue draft before the cold sets in. Standing-pilot furnaces are also simply old; most are past 15 years, and if yours needs repeated pilot repairs, our furnace lifespan guide and replacement timing help you weigh fixing against upgrading to a reliable electronic-ignition unit.
When to call a pro
Call a pro when a cleaned or replaced thermocouple still will not hold the pilot (the gas valve or venting is suspect), when the pilot flame is weak and yellow, or any time you smell gas. A thermocouple replacement runs $20–150, a pilot-orifice cleaning $75–200, and a gas-valve replacement $150–400. Because standing-pilot furnaces are aging out, ask whether repeated pilot problems are a sign it is time to replace the unit rather than keep patching a 20-year-old system.
Most of the causes above are maintenance problems wearing a disguise. OnOtto keeps the boring prevention on schedule so the 2 a.m. version doesn't happen.
Stop chasing symptoms, free for 30 daysFrequently asked questions
How do I relight my furnace pilot safely?
Turn the gas control knob to OFF and wait five minutes for any gas to clear, then sniff at floor level for the rotten-egg odor before you continue. Turn the knob to PILOT, hold it down, and light the pilot with a long lighter while keeping the knob pressed for 30 to 60 seconds so the thermocouple heats up. Release it, and if the pilot stays lit, turn the knob to ON. If you ever smell gas, stop, leave, and call the utility from outside.
Why does my pilot light go out only on windy days?
Wind points to a draft or venting problem rather than the thermocouple. A strong gust down the flue, a cracked vent pipe, or negative pressure from an exhaust fan can physically blow the small pilot flame out, and it only happens when the wind is up. Check the flue pipe for gaps or damage, and see whether a nearby kitchen or bath exhaust fan lines up with the outages. A tech can add a draft hood or reseal the venting so a breeze no longer kills the flame.
What is a thermocouple and can I replace it myself?
It is a thin metal rod, tipped in the pilot flame, that produces about 25 to 30 millivolts when hot and uses that signal to hold the gas valve open. When it fails, the valve closes and the pilot dies. It is a cheap part, roughly $10, and a handy DIYer can swap it: shut off the gas, unscrew the old one from the valve and its bracket, and thread in a matching replacement. If you are uneasy working around the gas valve, a tech does it for $20 to $150.
Should I replace a furnace that keeps losing its pilot?
Not for the pilot problem alone, since that is a cheap fix, but the age often behind it matters. Standing-pilot furnaces are usually 15 to 25 years old, well into replacement territory, and modern furnaces use electronic ignition with no standing pilot to fail. If yours needs a thermocouple, then a gas valve, then a flue repair in short order, you are patching a unit near the end of its life. Our furnace lifespan and replacement guides walk the age and cost math.