Signs and Symptoms of a Bad Furnace Thermocouple

The thermocouple is a vital furnace part that helps to keep you safe when the furnace is running for the season. Thermocouples are used on furnaces with standing pilot flames. The part shuts off the gas supply to the furnace if there is no standing pilot flame.


A thermocouple is a part made of two metal types. When one end of the thermocouple is heated, it creates an electrical charge at the end of the pilot light. When the part releases the charge, it triggers the gas valve to open and releases the gas flow to the burner.

Pilot Light

When the pilot light won't stay lit, it could be the gas flow or the thermocouple is bad and must be replaced. Press the gas control valve button to release some gas. Use a match to light the gas and produce a flame. Release the gas flow button. If the pilot light goes out, the thermocouple is bad or it could be dirty. If the pilot light doesn't light at all, it's the gas valve and not the thermocouple.

Low Flame

If the thermocouple doesn't keep the pilot light lit, it could be that the standing pilot flame is too low on the thermocouple. The pilot needs to heat the thermocouple with a blue flame that heats the top of the part. If the pilot flame is lower than seven-eighths of an inch from the top of the thermocouple, the pilot will go out. A dirty coupler will cause a low flame.

Dirty Thermocouple

Thermocouples get dirty from years of use and won't stay lit. Sometimes black soot from the pilot flame can clog the thermocouple port. Use a thin piece of wire to open the port on the thermocouple. If the soot is sounding the port, use an eraser or an Emory board to remove the soot. Sometimes the threads that connect the thermocouple to the gas valve become dirty and need to be cleaned so the parts can make a tight connection.