Pilot light won't stay lit: thermocouple, draft, or gas — telling them apart

For furnaces old enough to have a standing pilot, the same three suspects account for nearly every case.

Thermocouple (most likely)

The copper probe that sits in the pilot flame and holds the gas valve open. When it ages, the pilot lights but dies the moment you release the button, or minutes later. The flame should wrap the probe tip; a weak lazy flame or one blowing sideways starves it. Thermocouples are a ~$15 part and the most-replaced furnace component on earth — a confident DIYer can do it, but it is also a cheap technician visit.

Draft

A pilot that dies on windy days or when the blower kicks in is being blown out. Look for a missing furnace panel, a gap, or a flue problem letting gusts reach the pilot.

Dirty pilot orifice

A pilot flame that is small, yellow, and floppy instead of blue and firm has a partially clogged orifice. The flame cannot heat the thermocouple enough to hold the valve. Cleaning the orifice is delicate; many just have it done during a service visit.

Failing gas valve (least likely)

If a new thermocouple, clean orifice, and steady flame still will not hold, the electromagnet inside the gas valve is done. That is a real repair — price it against furnace age; on a 25-year-old unit this is usually the replacement conversation.

Safety line: Follow the relighting instructions printed on the furnace door exactly, including the wait time after a failed attempt — that wait is what lets unburned gas clear. If you ever smell gas while relighting, stop and ventilate.

Is the furnace flashing a light?

If the burners will not light, the control board is almost certainly flashing a diagnostic code behind the little window. Count the flashes and look the code up by brand here — walking into the phone call knowing the code saves diagnostic time.

No pilot? On newer furnaces this fault is a code instead

Standing pilots disappeared from new furnaces years ago, but the underlying fault — a real flame the furnace cannot prove — did not. The thermocouple's modern descendant is the flame sensor, and when its signal goes marginal the control flashes a weak-flame warning instead of letting a pilot die:

Brand	Code	What it reports
Armstrong / AirEase / Concord / Ducane	4 flashes	Low flame signal — check the flame sensor
Goodman / Amana	7 flashes	Flame signal is too weak
Goodman GMEC96 / Amana AMEC96	rapid amber flashing	Low flame sense current
Lennox	Alert code 240	Low flame current in run mode
Trane / American Standard	8 flashes	Low flame sense signal
York / Luxaire / Coleman	rapid amber flashing	Low flame sense current

These are warnings rather than failures — the Goodman and York pages both describe a furnace that still heats and will not for much longer. It is the same story a dying pilot tells: the flame is real, the proof of it is marginal, and the cheap fix is restoring the proof.

Fix it yourself, or call?

Homeowner territory on a standing-pilot furnace: relighting exactly per the door-panel instructions, including the full wait time after a failed attempt — that wait is what clears unburned gas. A thermocouple is a ~$15 part and the classic borderline job: confident DIYers do it, but it threads into the gas valve, so it is also one of the cheapest technician visits there is. When in doubt, it is a call.

Technician territory with no exceptions: the pilot orifice, gas pressure, and above all the gas valve. A valve that will not hold with a new thermocouple, a clean orifice and a steady blue flame is failing internally — never a homeowner part.

What the fix typically costs

On modern furnaces the equivalent fix — flame sensor cleaning — runs $80–200 in the repair cost guide, its classic ten-minute job. A failing gas valve is the other end of the scale at $300–900, never DIY. On an old standing-pilot furnace, weigh that against the replacement cost page's $3,500–8,000 straight swap: the repair-quote-times-age rule usually decides this one.

Error codes behind this symptom

If your furnace is flashing a code while doing this, these code families are the usual suspects: flame sensor codes, gas valve codes. Look your exact code up from the brand index, and see the furnace repair cost guide for what the fix typically runs.

Sources

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.