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A mass air flow sensor (MAF) is an electrical component designed to determine the mass of the air entering a modern fuel-injected engine. This information can allow the engine control module (ECM) to provide the correct amount of fuel to the fuel-air mixture. By tightly controlling the ratio of the fuel-air mix, combustion can be rendered more efficient while emissions are reduced. A faulty mass air flow sensor can cause an engine to run poorly, or even not at all.
The two primary types of mass air flow sensor are known as the hot and cold wire types and the vane meter variety. Though they differ in design and operation, they both attempt to determine the mass of the air passing through the sensor. Since the air intake system is tightly sealed, this can effectively tell the ECM the mass of the air actually entering the system. If air is introduced to the system in a post-sensor location, it can lead to a lean running condition due to there being more air than the ECM expects.
Vane meter sensors use a spring-loaded vane to estimate the mass of the air flowing past them. The vane can obstruct part, or all, of the air flow, and the mass is estimated based on how far it moves. This can have the undesired effect of restricting air flow to the engine, which is solved by other mass air flow sensor designs.
As their names indicate, both hot and cold wire sensors have a wire, which passes horizontally through the air flow. The hot wire variety uses an electrical current to heat the wire, and then measures the electrical resistance of the wire to determine the mass of the air flow. The wire is cooled off as more air flows over it, allowing for a fairly accurate estimate to be made. This solves the problem of obstruction, while being particularly susceptible to contamination from oil. The cold wire type uses a similar idea, though it has an oscillating electrical circuit to determine the air flow.
Mass air flow sensors are typically used in concert with a variety of other sensors to improve combustion efficiency and reduce emissions. Most modern engines will use one or more oxygen sensors in the exhaust. While the mass air flow sensor typically provides a good estimate of the air mass, monitoring the exhaust gases can allow the ECU to make small adjustments as necessary.