I have had the X5 for about a month now and the parking sensor hasn't worked since I got it. I would just get a continuous beep and a flashing pdc light when reverse selected. Did a scan with PC soft and it said all 4 rear sensors were crook. Highly unlikely I thought, probably more like wiring fault. Did some searching on the net and found this.
Sure enough, the plug was unplugged. Plugged it back in and lo and behold, PDC works. Can only guess it was the idiot that installed the reversing camera as there is single green wire that goes right past the socket and into the tailgate. Plug and socket located on the right side rear under plastic cover (under cargo floor) in right back corner.
Cheap fix