Hi I Recently purchased a 3.0TD6 2006 that had a suspension fault (kept saying normal height only)
I have replaced the compressor with a brand new one and cleared the fault codes. Everything has been fine for 3 weeks, now when i start the car it shows suspension fault on the display. The car rises and falls if i adjust the suspension and seems to drive Ok but the suspension fault shows up when i start the car.

Scanning the car show fault C1B18 Supply Voltage x-ref Check

Any ideas where to start looking to fix this?

First point may sound daft, after messing with the suspension, did you turn the steering full lock to lock to reset the system, I always do this twice, slowly and momentarily hold when at full lock. This has worked for me, after working on the suspension and getting fault messages.
Check the system on the screen, are all sensors indicating the same with no flashing signals.

After that I am guessing, unless you have a fault with one of the height sensors. not much sorry, but hope it helps.