Faulty Manifold Absolute Pressure Sensor
Manifold Absolute Pressure Sensor harness is open or shorted
Manifold Absolute Pressure Sensor circuit poor electrical connection
Faulty Mass Air Flow (MAF) Sensor
Engine Light ON (or Service Engine Soon Warning Light)
This fault code is stored in the engine control module when the actual boost pressure differs from the desired boost pressure. When this fault code is stored the ECM enters a limited operating strategy. This reduces the boost pressure to a minimum to limit the possibility of damage caused by excessive boost pressure.
Variable geometry turbocharger.
Most systems have a failsafe function. The turbocharger is in a position where it is unable to deliver any measurable boost pressure. When the engine starts, a depression, or vacuum, is applied to the turbocharger. This causes the vanes in the turbocharger to move to a position where maximum boost pressure can be delivered. As the boost pressure increases the depression applied to the turbocharger is reduced. This reduces the increase of boost pressure as the engine load increases. An electrical or vacuum system failure will result in a loss of boost pressure.
An over boost condition is normally caused by a malfunction within the turbocharger. This malfunction causes the vanes to become stuck in a position which causes the boost pressure to increase.
An under boost condition can be caused by a faulty turbo boost control solenoid, leaking vacuum pipes, or an electrical malfunction of the boost pressure control circuit.
If a vacuum-operated solenoid is used to control the system a blocked vent can also result in an over-boost condition.
Where a turbocharger control module is used an electrical malfunction can cause the actuator to get jammed in one position. Depending on where the actuator jams the boost pressure can be affected accordingly.