The Mass Air-Flow Sensor directly measures the mass of the air entering the engine. This is accomplished using either a flapper-valve which is deflected a certain angle by the mass of the air as it flows through it, or with a heated wire element in the air-stream that changes resistance based on temperature. The more mass of air that flows across the heated element, the more the element cools down, and thus the less resistance it measures. The Maf Sensor is quite different than the Map Sensor used in all PGMFI systems.