Hi Gogubjp .... they guys above are absolutely correct...
Looks like classic weak compressor... at least that is the cheapest place to start!
I bought the piston ring from bagpipingandy.... £25?
Don't buy anywhere else as the material is too hard with the other sellers - I tried once before I found Andy via here.
My one from Andy lasted initially about a year but my compressor wrecked the ring because it was scored inside.
Again Andy got me a new cylinder head (for compressor) £40....... amazing when you think that BMW Portsmouth want £1200 + VAT just for the compressor - they will not service them.
The car will not sink... if you have no leaks it should stay up always.
Disconnecting the compressor makes no difference to height - if the ignition is off there will be no pressure on the compressor side if yours inactive.
It took me about an hour first time I changed it. But only 20mins max once you have done it once - lets hope you only have to do it once
The new one has been in for over 2 years no probs - in fact it has more pressure now (14 Bars) ..... if your system drops below 9 bars it will not operate.
BPAndy has posted great videos on youtube & on his site... go slow & steady... he does it in about 4 mins flat !
Be bold - have a go or get the kit, then get Justin at E-Tech Motorsport Havant to do it for you.... (am I right in saying you are in the same town? PS: I am in Chichester area)... he is one of the few people who I have met who completely understands the system in the South - v honest guy - he did the levelling calibration on mine after another fault where mine sagged due to other issues.
Once the compressor ring is replaced the system will still not raise or lower until you wipe out those codes...
Drop me a line if any probs...
If you go to J tell him hi from me (Malc with the 4.8is in Selsey)
Best
M