See also:

Brake fluid reservoir
The brake fluid should always be above the MIN mark on the side of the reservoir. Check, without removing the cap, that there is sufficient fluid in the reservoir. Fluid type: DOT 4+ boiling poi ...

Call volume
Call volume can be only be adjusted during a call. Use the buttons in the steering wheel keypad or the infotainment system’s VOL control. ...

Low battery voltage
If the battery voltage is too low, the parking brake cannot be applied or released. Connect an auxiliary battery if the battery voltage is too low, see page 114. ...