" a friend who had the rear air ride sensors replaced "

The first thing you should check is that sensor, your friend might have reassembled it the wrong way...these sensor have to be is a specific position (long arm pointing upwards with a slight angle). Could be an easy and cost free fix...

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BMW X5 E53, 4.8iS - 2005
Professional Stereo/Nav with DSP (modified BM54 module) - Converted to Eonon GA5166X with Reverse cam
4-Corner Air Suspension - Converted to BC Coilovers