See also:

Problems with the SmartKey
Problem Possible causes/consequences and Solutions You cannot lock or unlock the vehicle using the SmartKey. The SmartKey battery is discharged or nearly discharged. ...

Cleaning the steering wheel and gear or selector lever
  Thoroughly wipe with a damp cloth or use leather care agents that have been recommended and approved by Mercedes- Benz. ...

Towing a vehicle with both axles on the ground
When having your vehicle towed, observe the important safety notes WARNING The power assistance for the steering and the brake force booster do not work when the engine is not running. You ...