To find the faulty one, turn on the ignition so the pdc is active then listen to each sensor you should here a ticking on the good ones and the bad one will be silent (you need to be in a quiet place) you can get the sensors out from the front, there 3 clips behind the head of the sensor which can be depressed to squeeze through the hole in the bumper, they may need bending back into shape before you replace the sensor. hope it helps

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2001 X5 4.4 Sport