PILOT OUT – On a “standing-pilot” furnace, the pilot light is not burning and the furnace will not come on.
POSSIBLE PROBLEM & DESCRIPTION
Many older gas furnaces utilize a “standing pilot” (a small pilot flame that burns continuous inside the burner compartment).
Standing pilot furnaces require a thermocouple to ensure that the pilot is lit before allowing the main gas valve to open.
If a standing pilot furnace will not come on, check to see if the pilot is burning.
If the pilot flame goes out, you may try relighting it. If it will not stay lit, the most likely cause is a bad thermocouple.
Dirty Pilot Orfice - Qualified Technician Recommended
In order for a thermocouple to properly function, it must have a sufficient heat source to activate. If the pilot orfice becomes clogged, the pilot burner may not be able to produce enough heat to activate the thermocouple.
The pilot flame should be strong enough that the flame surrounds the sensing element of the thermocouple.
Bad Gas Valve - Qualified Technician Recommended
If the thermocouple is good and the pilot flame is sufficiently burning but the pilot will not stay lit, there is a possibility that the main gas valve is faulty.