VANOS is the BMW system for variable valve timing and it uses two solenoids to control that. These solenoids move back and forth and direct oil through small passages inside the cylinder head that ultimately cause the cam phasers to retard or advance. These solenoids get oil varnish built up inside of them and start to stick and so it's possible that cleaning them with solvent might solve the problem.

If cleaning or replacing your vanos solenoids doesn't ultimately solve your problem, check out this video for even further details on this issue:

E90 Cam Bearing Ledge Failure:
   • This causes VANOS codes! - E90 Cam Be...  

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