I had a problem with a PDC sensor on a 2001 X5. Turned out to be a faulty sensor. But there is a way to figure out which one(s) is (are) the problem.

Turn on the ignition and shift into reverse (without engine running) and put your ear up next to each sensor. You can hear a faint ticking sound from each functioning sensor. A faulty sensor will be silent. Of course it helps if there isn't much ambient noise because they are hard to hear. But once you know what to listen for, it's pretty easy to find the faulty sensor(s).

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2002 X5 4.6is - Imola Red/Black (Sold)
2001 740i M Sport - Anthrazit Metallic/Black (Sold)
1998 M3/4/5 - Technoviolet/Dove (Sold)
2003 Z4 3.0i Sport - Sterlinggrau Metallic/Black (Sold)
2009 Mini JCW (The Wife's)
2015 i3 REx - Arravani Gray/Tera World (Daily Driver)