Fixing your Air Suspension. ↗
Owner guide to the 'Self Leveling Suspension Inactive' fault. Clear the code with a scanner so the compressor runs again, then check the likely culprits in order: leaking air bags, a worn compressor (rebuild kits with a new piston ring and seals take about an hour), the ECU, and the reservoir and fittings (soapy-water leak test).
Tools mentioned: Scan tool that reads EHC faults, 1/4 in socket set, Torx bits
Written on an X5 4.8is (two-axle system); compressor advice applies to the rear-only system too.