Good evening i will try to be as helpful as possible.The vanos mechanically stuck fault can be a few things which can be difficult to diagnose.First was anybody inside the engine,because if the timing is off, this fault can be stored.Another possibility is that the vanos solenoid may be stuck,and i have included instructions for checking the solenoid.The other possibility is that the vanos unit itself is faulty.It may be dirty, worn out and just not moving correctly.Hope this helps and good luck- feel free to reply anytime after accepting this answer-
BMW Technician
bmw factory trained since 2000 current shop foreman