The vehicle’s computer relies on the information provided by the various sensors and actuators available. Its purpose is to make the necessary adjustments to obtain the optimum air/fuel mixture for the engine. The values monitored by the Engine Control Module (ECM) are the airflow, engine speed, atmosphere temperature, coolant temperature, among others.
The Barometric Air Pressure Sensor (BAP) is used to measure the atmospheric pressure. This value is necessary to determine the air/fuel mixture because the ECM compares the atmospheric pressures with the intake manifold pressures to accurately adjust the mixture.
The setting of the P2228 OBD2 diagnostic code is due to the ECM detects a lower than the expected electrical value in the BAP sensor or its circuit. In this case, the description “A” may refer to a specific circuit. It happens the same when it has to do with multiple sensors.