Necessary preliminary tasks:

Read out fault memory of control unit of Digital Motor Electronics (DME).
Switch off ignition.
Remove acoustic cover.
Remove design cover.
Remove upper section of intake filter housing with air-mass flow sensor and intake hose. Refer to 13 54 250 REPLACING AIR INTAKE HOSE (BETWEEN air-mass flow sensor and throttle valve assembly) (N62) , 13 62 560 REMOVING AND INSTALLING/REPLACING AIR-MASS FLOW SENSOR (N62) and 13 72 001 REPLACING AIR CLEANER ELEMENT (N62) .

NOTE: Solenoid valve (1) controls adjusting unit on inlet side.
Solenoid valve (2) controls adjusting unit on exhaust side.

Fig 1: Identifying Solenoid Valves

Courtesy of BMW OF NORTH AMERICA, INC.

Unlock plug of solenoid valves (1 and 2) and disconnect.

Release screws and remove holder (3). Pull out solenoid valves (1 and 2).

Installation:

Check holder (3) for deformation; replace if necessary.

Installation:

Clean sealing faces (1) on cylinder head.

Fig 2: Cleaning Sealing Faces On Cylinder Head

Courtesy of BMW OF NORTH AMERICA, INC.

Installation:

Replace O-rings (1 and 2) on solenoid valve (3) and lubricate.

Fig 3: Identifying O-Rings On Solenoid Valve

Courtesy of BMW OF NORTH AMERICA, INC.

Installation:

Press in solenoid valves as far as they will go.

Make sure holders are in correct installation positions.

Install and tighten down screws.

Now clear the fault memory.

Fig 4: Installing Solenoid Valves

Courtesy of BMW OF NORTH AMERICA, INC.