The PCM (Powertrain Control Module) uses a pulse control solenoid to control the amount of boost pressure, which is generated by the turbocharger. The PCM controls the pressure via a pressure sensor in the turbocharger. This sensor is a variable resistor, which has a 5-volt reference signal that is given by the Powertrain Control Module (PCM).
When the boost pressure is low, the resistance of the sensor is high, resulting in only a small percentage of the reference signal being returned to the PCM. As the pressure increases, the resistance decreases. Thus, more of the reference signal returns to the PCM. This voltage variability is used by the PCM to calculate the actual boost pressure. The value of zero boost pressure is equal to approximately 0.5 volts. While the maximum boost is approximately 4.5 volts.
When the PCM detects that a boost pressure voltage exists, which is higher than 4 volts at a time when it was ordered unboosted, it sets the DTC code P0238 OBDII there.