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.