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Morning John It depends what Diagnostic he was using there are several systems that can communicate with the car but mainly the key control units. Foxwell, Autologic, iCarsoft there is quite a long list then there is BMW ISTA/INPA which can see every component, it includes Workshop Information (WIS) which gives Photos and descriptions of how to do the job plus advice on test programs to confirm diagnostics. I use ISTA at home The air suspension system isn't complicated it has 2 Air Springs 2 Ride Height sensors 2 Fuses a Relay a Control Unit (Programable) and the Compressor. As your issue started with the Rear Ride Height sensor I would begin there with the Plug Connector and wiring make sure there is no damage to the wiring. You will need a Wiring Diagram (ISTA) each plug has 3 wires that feed back to the Control Unit (the Control Unit is in the Rear Right Cubby at the bottom of the Fuse board) BMW rear air springs are used on several models not just X5's I have seen several where the wiring had become cracked and brittle with broken connections. That is where I would start, if continuity is there then in my experience Control Unit failure is rare but not impossible a replacement from a Dismantler would be a starting point and not to £££ they are plug and play. With ISTA they can be checked and recalibrated if needed. Dave