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In case of a fault related to VANOS/Camshafts. Few known causes can be checked:

• Dirty or faulty VANOS Solenoids: The most common cause, where the exhaust solenoid sticks, jams, or fails completely.
• Dirty oil/Low Oil Pressure: Old or incorrect oil affects solenoid operation. The oil being a hydraulic medium for the VANOS must always be clean and compliant
• Camshaft Position Sensors: Faulty sensors causing incorrect timing readings. They get demagnetized with age or coated with metal/gunk debris
• VANOS Check Valves: Clogged non-return valves in the engine block. Especially the exhaust one.
• Camshaft Ledges or Adjuster: Worn camshaft bearing ledges (especially on early N52s prior to 2008) or worn adjuster gears.

FCP Euro has a good article on the camshaft bearing ledge issue on high mileage or poorly maintained N52 engines.