The following steps should always be carried out in the specified order before working in the engine compartment :

Park the vehicle on a level and stable surface.

Depress and hold the brake pedal until the engine has stopped.

Apply the handbrake firmly Braking, stopping and parking Brakes Parking .

Select the neutral position or move the selector lever to P Changing gear Changing gear Automatic gearbox see Automatic gearbox_0 Automatic gearbox see Changing gear_0 Automatic gearbox Manual gearbox see Changing gear_0 Manual gearbox .

Stop the engine and remove the key from the ignition Starting and stopping the engine Engine and ignition Ignition see Engine and ignition_6 .

Allow the engine to cool sufficiently.

Children and other people should be kept well away from the engine compartment.

Ensure that the vehicle cannot roll away unexpectedly.

WARNING
Preparing the vehicle for working in the engine compartment
Opening and closing the bonnetTowing Recognition Control Module -J345
General Information
The Towing Recognition Control Module -J345- recognizes
“trailer mode” when there is a power decrease of at least 5 W
→ Note and transmits this information to various control
modules via the CAN data bu ...
Battery, Testing using Midtronics Battery Tester -MCR340VKT
Requirements
WARNING
Read and follow the Safety Precautions when working
with batteries. Refer to
→ Chapter „Warnings and Safety Precautions“.
...
13-Pin« Trailer Socket
The backup lamps, »continuous current«
and charging cable can be used on the trailer with the
»13-pin« trailer socket. A
»13-pin« trailer socket is
recommended if a bicycle rack will be installed or a camper will
be towed.
  ...