A thermocouple is the safety sensor on an older standing-pilot furnace — the kind with a small pilot flame that burns continuously. The tip of the thermocouple sits in that pilot flame and, through a small voltage the heat generates, holds the pilot gas valve open. If the pilot ever goes out, the thermocouple cools, the voltage drops, and the valve closes to stop gas from escaping. It is the standing-pilot equivalent of the flame sensor on a modern electronic-ignition furnace.
The classic symptom of a failing thermocouple is a pilot that lights but won't stay lit once you release the gas knob — and, consequently, no heat, because the main burners can't run without a proven pilot. The usual causes are a worn-out, dirty, or misaligned thermocouple tip, or a pilot flame burning too weak or off-center to keep it hot. Most thermocouples last eight to twelve years.
This is one of the least expensive furnace repairs. The part is $10 to $20 and installs in 15 to 30 minutes; a professional visit typically runs $100 to $300, often $125 to $180, mostly the service call. Thermocouples only apply to aging standing-pilot equipment — if your furnace has one, the system is likely decades old, which makes the thermocouple repair a good moment to weigh a small fix against the efficiency and safety gains of a newer unit rather than an automatic reason to replace.