CITROEN DS3 Handbook page 130

Rear parking sensors
System consisting of four proximity sensors,
located in the rear bumper.
This detects any obstacle (person, vehicle,
tree, gate, etc.) behind the vehicle, except
obstacles located just below the bumper.
An object, such as a stake, a roadworks
cone or any other similar object may
be detected at the beginning of the
manoeuvre, but may no longer be detected
when the vehicle comes close to it.
Activation
The system is activated as soon as reverse
gear is engaged, accompanied by an audible
signal. The proximity information is given by:
- an audible signal, which becomes more
rapid as the vehicle approaches the
obstacle.
- a graphic (according to version) in the
multifunction screen, with blocks which
become increasingly closer to the vehicle.
The parking sensor system cannot in
any circumstances be a substitute for
vigilance on the part of the driver.
When the distance between the rear of the vehicle
and the obstacle is less than thirty centimetres, the
audible signal becomes continuous and the "Danger"
symbol appears in the multifunction screen.
On completion of the manoeuvre, when you exit
reverse gear, the system becomes inactive.