Self level susp inact - fault code 57 23

[Jaksi]

Hi guys,

hope that someone is able to help me with my problem related to
self levelling system (only in rear axle) in my 2001 4.4i X5.
Compressor unit wont start and every time when start my x i get alarm to instrument panel saying self level susp inact, however x is standing straight and there is no leakage in system, all fuses seems to be OK (30A & 7,5A if i remember correctly).

We have checked fault codes and are getting fault code 57 23.
I hope that someone can tell what this code means and which parts should be checked.