Mass Air Flow sensor. Measures the mass of air entering the engine intake. Used by ECUs to calculate fuel injection quantities.
MAF sensor (Mass Air Flow sensor) is an engine sensor that measures the mass of air flowing through the intake. The sensor uses a hot-wire or hot-film element heated to a specific temperature; as air flows over the element, it cools, and the sensor measures the cooling rate to calculate mass airflow. The ECU uses this measurement to determine fuel injection quantities and timing.
MAF sensors are common in production cars, particularly American and Asian designs. The advantage of MAF sensing is direct measurement of airflow, which is more accurate than indirect methods. The disadvantage is sensitivity to contamination (oil from oily air filters, dust, debris) and the requirement for the sensor to be in the intake stream, which can restrict airflow on heavily modified engines.
Aftermarket MAF sensor relocations or replacements are common in tuning. Common modifications include relocating the MAF to a less-restrictive location, replacing the stock MAF housing with a larger one (allowing higher airflow), or eliminating the MAF entirely in favor of MAP (manifold absolute pressure) calculation. K&N filter products are sometimes blamed for MAF contamination because oil-coated filters can foul the hot-wire element.