Mercedes-Benz W220 common rail diesel goes into limp mode: boost pressure and air mass
Why does W220 CDI power drop and the boost pressure sensor fault code stay on?
The cause can be the sensor's wiring harness: the wire sheath can be broken and the wires touch each other. The harness should be checked under load, because the fault does not always show in a measurement made at rest. After the wires were repaired and insulated, the codes disappeared.
What does fault code P0236 mean in the W220 CDI?
P0236 refers to the range or performance of the turbo boost pressure sensor A circuit. The sensor test does not always reveal the fault, so more information can be obtained from the freeze-frame data in a workshop's diagnostic tool.
What else can make the mass air flow sensor readings too low?
A blocked air intake. Paper had got into the turbo intake pipe, preventing airflow, and removing the paper fixed the fault. Check the intake pipes before replacing the sensor.
Does the vacuum pump affect boost pressure?
Boost pressure is controlled by vacuum, so a fault in the vacuum pump can affect the turbocharger. The same pump assists the brakes, and when the pump fails the brake pedal gets harder. The brakes are safety-critical, so have a workshop check it.