Furnace Pilot Light Going Out

The thermocouple is a small metal probe that sits in the pilot flame. The heat generates a tiny voltage that tells the gas valve 'pilot is lit, keep gas flowing.' When it fails or oxidizes, the voltage drops, the valve closes, and the pilot goes out.

Light the pilot per the furnace label. Hold the knob down for 60 seconds, then release. If pilot goes out within seconds, thermocouple is failing. Visual check: probe should sit IN the flame, not next to it.

The tiny opening that lets gas through to the pilot has accumulated dust, spider webs, or corrosion. Reduced gas flow makes a weak yellow flame that can't heat the thermocouple properly.

Pilot flame is yellow, orange, weak, or flickering. Should be a steady blue ~1 inch tall fully engulfing the thermocouple tip.

A nearby air leak, return duct, or improper combustion air supply creates drafts strong enough to extinguish the pilot. Most common after duct work or basement renovations.

Light a stick of incense near the furnace and observe smoke movement. Significant air movement near the burner area = draft issue.

Gas supply pressure has dropped below what the furnace requires. Could be utility-side issue, a partially-closed valve, or a regulator problem.

Other gas appliances also weak (water heater takes forever, stove burners weak)? If yes, utility issue. If only the furnace pilot is weak, the regulator at the furnace may be the problem.

The gas valve's pilot safety mechanism has failed. Even with a good thermocouple signal, the valve won't hold the pilot gas open.

Confirmed by elimination — new thermocouple, clean orifice, no drafts, but pilot still won't stay lit. Or audible clicking from the valve when releasing the pilot knob.