The Manifold Absolute Pressure (MAP) Sensor is used to measure engine load. It receives a reference signal from the PCM (Powertrain Control Module), which changes according to the vacuum of the manifold. This information is used by the PCM to adjust the ignition time and fuel delivery. The vehicle may use a Barometric Pressure Sensor (BARO), which is completely independent. This device only measures barometric pressure and is similar to the MAP Sensor in design, but is much more sensitive to changes in atmospheric pressure.
The PCM examines the voltage of both sensors at idle, in case it finds a difference. If there is a large difference at the input of these sensors, the DTC code P0069 OBDII is set. It is also worth mentioning that the PCM observes the voltages of these sensors to determine if they are correct when the accelerator is in the open position.