Furnace pilot light keeps going out

A furnace pilot light that keeps going out usually has a worn or misaligned thermocouple, a dirty pilot opening that gives a weak yellow flame, or a draft blowing the flame out. The thermocouple is the safety device that shuts off the gas when it does not sense the pilot. One relight following the label is reasonable. Repeated outages need a technician.

Updated November 2026 · General guidance, not an on-site diagnosis

Safety first. If you smell gas, do not try to relight the pilot. Leave the house and call your gas utility or 911 from outside. Never hold the pilot button down with tape or a weight, and never bypass a thermocouple.

Check these first

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Confirm the furnace actually has a standing pilot. Newer furnaces use electronic ignition and have no pilot to light.
Check that other gas appliances are working. If none are, call your gas utility.
Look at the pilot flame through the viewing opening without touching anything. A steady blue flame is healthy. A weak, yellow or wavering flame is a fault.
Look for drafts near the furnace: an open window, a dryer vent leak or a gap in the furnace closet door.
Read the lighting instructions on the label inside the furnace's access panel before doing anything else.

Still not working after these? That is the point to bring in a licensed technician.

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What it usually means

Older gas furnaces keep a small flame burning all the time to light the main burners. Next to it sits a thermocouple, a metal probe that produces a small voltage when the pilot heats it. That voltage holds the gas valve open. When the pilot goes out, the thermocouple cools and the valve closes. The furnace is protecting you, and the cause is nearly always the pilot flame or the thermocouple.

A pilot that goes out once after a windy night or a gas service interruption can simply be relit. A pilot that goes out again within hours or days has a fault.

Likely causes, most common first

Worn thermocouple. The probe weakens with age and stops producing enough voltage to hold the valve open, even with a good flame.
Dirty pilot opening. Dust and soot partly block the small opening. The flame turns weak and yellow and no longer wraps around the thermocouple tip.
Misaligned thermocouple. The tip has shifted out of the flame.
Drafts. Air movement from a nearby vent, a leaky flue or a gap in the cabinet blows the flame out.
Gas supply or pressure. Low pressure, a low propane tank or a failing gas valve.
Venting problems. A blocked or backdrafting flue can push exhaust back down and snuff the pilot. This one is a carbon monoxide risk.

Relighting a furnace pilot light

Every furnace with a standing pilot has lighting instructions printed on a label, usually inside the access panel. Follow that label exactly, because the steps and the waiting time differ between models. The label will tell you to turn the gas control to Off and wait for any gas to clear before you light anything. Do not skip that wait.

If you smell gas after waiting, stop and leave the house. If the pilot will not light, or lights and goes out when you release the button, stop after one attempt. Holding the button longer or trying over and over does not fix a bad thermocouple. If you are not comfortable with any of it, a technician can relight the pilot and test the system on the same visit.

What a technician will do

The technician measures the thermocouple's output with a meter, cleans the pilot assembly, adjusts the flame so it covers the thermocouple tip, and checks gas pressure. A thermocouple is a small, low-cost part and replacing it is a short job. The technician should also check the flue and draft, because a pilot that keeps blowing out can be the first sign of a venting problem.

When it points to replacement

A thermocouple is a repair. If the technician also finds rust, a damaged heat exchanger or a failing gas valve, pricing a new furnace is reasonable. Newer furnaces light with an ignitor and have different failure points, covered in furnace not igniting and the furnace repair guide.

Common questions

How to light the pilot light on a furnace?

Use the lighting instructions on the label inside your furnace's access panel. They are specific to the model. In general they have you turn the gas control off, wait for gas to clear, switch to Pilot, hold the button while lighting, then release and turn to On. If you smell gas, do not light anything.

What does the thermocouple on a furnace do?

It is a heat-sensing safety probe that sits in the pilot flame. While the pilot heats it, it generates a small voltage that keeps the gas valve open. When the pilot goes out it cools, the voltage drops and the valve closes, which stops gas from flowing with no flame.

Is a furnace pilot light out dangerous?

With a working thermocouple, no. The gas valve closes when the pilot goes out. The danger comes from a gas smell, from a venting problem that keeps blowing the pilot out, or from bypassing the safety. If you smell gas, leave and call the gas utility from outside.

What color should a furnace pilot light be?

A healthy pilot is a steady blue flame, sometimes with a small yellow tip, that wraps around the end of the thermocouple. A mostly yellow, lazy or flickering flame means the pilot opening is dirty or the air mix is off, and it should be cleaned and adjusted by a technician.

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