In this entry – as if very simple system: PDC.
I’m sure – each BMW user has encountered PDC system problems if the car is equipped with it. Sensors of the PDC system are exposed to the environmental impact (water, salt, stones); swift temperature changes; the impact of the vibration. There is no surprise when any of the sensors of the wire connection fails.
There are diagnostics specialists who are using the method of listening, which of the sensors is (not) ticking. There are more advanced diagnostics specialists who check the error message list. Unfortunately, the error message regarding any of the sensors is not a certain indication of the cause of the defect. There could be several causes – damage to the sensor itself, damaged wires, or connectors.
I’m voting for rational work (probably, I’m lazy), so I maximally use options given by the diagnostics to identify the cause of the problem more exactly. If you understand the cause of the problem precisely, you don’t have to take off the bumper unnecessarily. Of course, the sporadic or especially cunning defects, but this time I got lucky,

But now – from the beginning.
When checking the error message list with INPA, the active error message is seen immediately:

What happened to the inside sensor on the left side, on the front? Is the contact lost? Defect of the sensor?
To evaluate the situation, I open the sensor data. I start with a distance:

As we see, all sensors indicate the maximal distance (no physical barrier), so in this case, for example, the dirt on the sensor.
A more interesting menu is oscillation data of the sensors, which you see in the next menu. The sense of this menu is following:
a. the energy impulse is supplied to the ceramic plate (resonator of the sensor), the plate in self-resonant mode starts to fluctuate and “sends” the packet of the ultrasound impulse. The bearing frequency is above the diapason (20.000 Hz), which we can hear, but this sound perfectly reflects from all objects;
b. if the sound reflects from some object, the plate perceives the signals. As the frequency of the receiving signal is the same as the self-resonation frequency of the plate of itself, exactly this sensor is with high sensitivity, but it is efficiently filtering other signals – different frequencies, which are “in the near”.
Here, this menu indicates how good the main element of the sensor – resonator of the ultrasound – “works”. If the sensor does not resonate or badly resonates in the “transmission” mode, there is no reason to hope that it will receive any signal. The reason is simple – it is not possible to receive a signal if it is not sent.

Here we see, that the oscillation time of the front sensor, inside on the left, is significantly shorter than the minimum allowed. At the same time – bar indications are changing rapidly, so – the data of the sensor are received on a regular basis. What could be the reason? For example, damaged resonator – microcracks in it; current leaks because the saltwater (it conducts the electricity well) has got inside the sensor.

Pay attention to the left side inside sensor on the front part. Its oscillation time is at the minimum allowed limit. What happened to this sensor?
A year ago, this sensor stopped working. The Covid had just started, at the specialized service new OEM sensors were not available. Delivery times – unclear. So for the experiment, the sensor of an alternative manufacturer was installed, with no intentions for a long lifespan. Now one year has passed, as we see – the sensor is close to the condition when the error messages will be written.
Obviously, the ultrasound sensors of the alternative manufacturers are of lower quality (with a less electrical value Q or the electronics elements are with higher current leaks), which gives faster calming of the fluctuation and… The error messages regarding the performance of these sensors after a quite short time of their exploitation are well known.
This time is clear, that everything is fine with the PDC module itself. Everything is fine with wires and plugs. Two PDC sensors have problems, so they have to be replaced.