A manifold absolute pressure sensor is located on the intake manifold of a vehicle's engine. Inside the MAP sensor is a resistor that moves in relation to the intake manifold pressure.
Fuel-injected vehicles typically have a MAP sensor to provide instant manifold pressure to the engine's electronic control unit. The sensor calculates air density and determines the engine's air mass flow rate. The electronic control unit uses this information to determine how much fuel to use for optimum combustion and to influence the advance or retardation of ignition timing. Some fuel-injected engines use a mass airflow sensor, or MAF, to measure airflow instead.