Had the car in for diagnosis of "SELF LEVEL SUSP INACT" message which reappeared after about 350 kms since it was last reset during the Inspection II service.
Again no faults showing and no obvious defects found in the system.
My Tech suspects the air pump may be the culprit, but could not prove this conclusively enough to warrant the expense of replacing the unit.
Reset completed and I'll keep driving it, pending a return of the dreaded message.
The car rides and handles so much better with the system active again, but having that niggling intermittent issue lurking in the background annoys me so I may look at buying a new pump.
Does anyone know where I can get a good deal on a new air compressor, P/N 37226787617?
I see that Pelican have a Wabco unit for $USD808.00, while the genuine BMW unit is $1167 (I suspect this may be manufactured by Wabco anyway).

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Current:
2004 X5 4.8is - Le Mans blue/Cream beige
1994 E31 850CSi - Sterlingsilber

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1987 BMW E28 M5 - Delphin-metallic
1976 BMW E3 3.0Si - Arctic-metallic