The DIAFCS (Diesel Intake Air Flow Control System) is usually located screwed to the intake manifold in the intake airflow. This system controls the quantity of inflowing air through changing a signal to the engine driven by the PCM (Powertrain Control Module). The engine is responsible for opening and closing a throttle plate that regulates airflow.
The PCM is informed of the amount of clean, filtered air entering the engine, thanks to data provided by the MAF sensor. When the Air Flow Control System is activated, the PCM should notice a change in flow. If not, it will be considered an erratic operation.
When the P02E1 OBDII diagnostic code is stored, it is due to the detection of performance problems within the Diesel Intake Air Flow Control.