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Mass Air Flow Sensor

Mass Air Flow sensor (MAF) is used to measure the mass flowrate of air entering a fuel-injected internal combustion engine.

The air mass information is necessary for the engine control unit (ECU) to deliver the correct amount of fuel to the engine. Air changes density as it expands and contracts with temperature and pressure. In automotive applications, air density varies with the ambient temperature, altitude and the use of forced induction, which means that mass flow sensors are more appropriate than volumetric flow sensors for determining the quantity of intake air in each cylinder. When a MAF sensor is used in conjunction with an oxygen sensor, the engine's air/fuel ratio can be controlled very accurately by the engine control unit (ECU).

https://en.wikipedia.org/wiki/Mass_flow_sensor