Mercedes-Benz W212 parking sensor faults and sensor replacement
Why is the W212 parking system off with the red lights on?
The red lights indicate that the whole sensor system is out of use because of a sensor fault. Even one broken sensor is enough. The fault can be read with a diagnostic tool, which shows the number of the faulty sensor.
How is the faulty sensor found?
The diagnostic tool shows the faulty sensors by number. By hand you can check by covering the sensors one at a time: a working sensor clicks when pressed. Also check the connectors for moisture and oxidation.
What kind of sensor does the W212 need?
The sensor must be the original type, because a wrong sensor gives a fault code for the wrong component. At the rear all use part number A2125420018. In the front bumper the outermost ones use A2125420118 and the others A2125420018.
Can a rear parking sensor be replaced without removing the bumper?
Often yes. First disconnect the sensor wire, press the locking tab at the end of the sensor down and slide the sensor out towards the front of the car. The middle sensor may require removing the bumper mounts and pulling the bumper out at the sides so a hand fits.