Blind Spot Assist and Active Blind Spot Assist use radar sensors to monitor the area up to 40 m behind and 3 m next to your vehicle.
The system can detect vehicles travelling from speeds of approximately 12 km/h and issue a warning if they move into the monitoring range.
Grey: the system is activated but inoperative. - Green: the system is activated and operational.
If a vehicle is detected at speeds above approximately 12 km/h and this vehicle enters the monitoring range immediately afterwards, the red warning lamp in the corresponding outside mirror lights up red. In the Assistance menu, the lamp in outside mirror 
also lights up red, and the lane in which the vehicle is detected is hatched out.
If a vehicle is detected in the monitoring range and you switch on the turn signal indicator in the corresponding direction, a double warning tone sounds once and the warning lamp flashes red in the corresponding outside mirror. Red radar waves 
are displayed next to your vehicle in the Assistance graphic.
If the turn signal indicator remains on, the display in the outside mirror flashes for all other detected vehicles, but no further warning tone sounds. If you overtake a vehicle quickly, no warning is given.
Observe the notes on driving systems and your responsibility; you may otherwise fail to recognise dangers More.
Blind Spot Assist does not react to vehicles approaching and overtaking you at a greatly different speed.
Blind Spot Assist cannot warn drivers in this situation.
The exit warning is an additional function of Blind Spot Assist and Active Blind Spot Assist and can warn vehicle occupants attempting to leave a stationary vehicle about approaching vehicles.
The exit warning neither reacts to stationary objects nor to persons or road users approaching you at a greatly differing speed.
The exit warning cannot warn drivers in these situations.
If a vehicle is detected in the monitoring range, the red warning lamp lights up in the corresponding outside mirror.
If a vehicle occupant pulls the door handle on the side of the warning, a warning tone sounds twice and the ambient lighting in the respective door and the warning lamps in the corresponding outside mirror flash red.
Vehicles with ambient lighting: the Warning assistance of the ambient lighting can be activated and deactivated More.
The warning assistance can differ depending on the equipment and may vary according to the setting.
The exit warning is only available when Blind Spot Assist is activated and up to a maximum of three minutes after the vehicle has been switched off. The exit warning is no longer available once the warning lamp in the outside mirror flashes three times.
The exit warning is only an aid and not a substitute for the attention of vehicle occupants. The responsibility for opening and closing the doors and for leaving the vehicle remains with the vehicle occupants.
if there is dirt on the sensors or the sensors are obscured
in poor visibility, e.g. due to fog, heavy rain or snow
if there are narrow vehicles, e.g. bicycles or motorbikes
if the road has very wide or narrow lanes
if vehicles are not driving in the middle of their lane
Warnings may be issued in error when driving close to crash barriers or similar continuous lane borders. Always make sure that there is sufficient distance to the side for other traffic or obstacles.
Warnings may be interrupted when driving alongside long vehicles, for example lorries, for a prolonged time.
Blind Spot Assist and Active Blind Spot Assist are not operational when reverse gear is engaged.
Blind Spot Assist and Active Blind Spot Assist are not operational if transport equipment, for example a trailer or bicycle rack, is attached to the trailer hitch and the electrical connection has been correctly established.
when the sensors are covered by adjacent vehicles in narrow parking spaces
when people approach the vehicle
in the event of stationary or slowly moving objects
You can watch an animation on this topic via the following link:
