The Powertrain Control Module (PCM) monitors the boost pressure through the Sensor Manifold Absolute Pressure (MAP).
The MAP sensor has a 5-volt reference signal, which is supplied by the PCM, then the MAP Sensor directs a variable voltage signal to the Powertrain Control Module (PCM). If the boost pressure is high, the voltage signal is also high. If the boost pressure is low, the voltage signal is also low. Also, the PCM uses a solenoid to control the amount of boost pressure produced by the turbocharger. The MAP sensor also verifies that the boost pressure is right.
If the PCM detects a voltage signal indicating low boost pressure, even though a high-pressure command has been sent to boost control solenoid “A”, the DTC code P0237 OBDII is set.