Air suspension fault

My air suspension has now developed a fault. there's no way its ending up on coils(!) so I need to find out what's wrong. The symptoms: Start the RR, air suspension rises to middle setting. Followed by the high ride height warning lamp on the dash and the high ride height & low ride height button flashing repeatedly for a few seconds then going to a solid state. At higher speeds the system remains at middle height and the system isn't adjustable by the usual methods. Is there a way to diagnose the fault via reading codes? I have performed this task for an ABS fault but never for air suspension. Any help would be gratefully received!