Hi
Since I have my front bumper off I can refere to my experience and give some examples of how to detect problems with the ParkTronic device.
The front parking system is attached to a main connector found in front of the left wheelhouse. Check the connection. The two following pictures shows where to find it.
There are six sensors at the front, each one with its own connection to the main wire / cable. Each sensor have four pins in the connector. Check every connection, look for corrosion or weard / cracked wires. See next three pictures.
If you use an electric measuring device, and measure on the two "upper" pins, you get a value of 6.00 ohm at 20k, (0.6 at 200k). I tested all my sensors and they hade the same value / result. This might be a help to exclude if there is a faulty one. You can also connect the whole lose cable to the main connector in front of the wheelhouse ( swenglish ? ) and meassure / test if there is power to each sensor.
This might be a help. I guess the rear parking system is built in the same way.
Best regards Tomas