What effect does a malfunctioning thermocouple have on a gas furnace?

A malfunctioning thermocouple directly affects the safety and operation of a gas furnace. The thermocouple is a safety device that ensures the pilot light is lit. Its primary function is to detect the heat from the pilot flame and produce a small voltage; if the pilot light goes out, the thermocouple cools down and stops producing this voltage. As a result, the gas valve closes, preventing gas flow to the burner. This is an essential safety feature to avoid the risk of gas accumulation, which could lead to dangerous situations.

Therefore, when the thermocouple malfunctions, it may fail to detect the pilot light being out, leading to the gas valve remaining open, or it could not allow the furnace to operate at all. In either case, the proper functioning of the thermocouple is critical for safe operation, and if it is defective, the furnace will indeed fail to start when the pilot light is out. Thus, the correct answer highlights a fundamental aspect of furnace safety and operation related to the thermocouple's role.