What is the primary function of the flame sensor in a gas furnace?

The primary function of the flame sensor in a gas furnace is to detect the presence of a flame. This component plays a crucial role in ensuring the safe operation of the furnace. When the gas burner ignites, the flame sensor identifies the flame and sends a signal to the control board. If the sensor does not detect a flame when gas is flowing, it will shut off the gas supply to prevent the accumulation of unburned gas, which could lead to an explosion or fire hazard. Therefore, the flame sensor is critical for safety and efficiency, as it helps monitor the combustion process and prevent potential dangerous situations.

Temperature measurement, gas ignition, and airflow adjustment are functions typically managed by other components in the furnace system. The temperature is monitored by other sensors, ignition is managed by the ignition system, and airflow is adjusted by ductwork and blower mechanisms. Thus, the flame sensor is specifically tasked with flame detection, making it the right answer.