If you have a BMW with Park Distance Control (PDC), you may have had to replace the PDC modules at some point. Here's how you can test them to see if they're working properly.
To test the front PDC module, first put your car in reverse and activate the PDC. Then, place your hand behind the front bumper and wave it back and forth. You should hear a beeping sound as the PDC sensor detects your hand.
To test the rear PDC module, do the same thing but place your hand behind the rear bumper instead. Again, you should hear a beeping sound indicating that the rear PDC is working properly.