CITROEN DS3 Handbook page 160

Operation
When your vehicle is too close to or
approaches the vehicle ahead too quickly the
system automatically applies the brakes to
avoid a collision.
You are then alerted by the display of a
message.
The brake lamps on your vehicle come on to
warn other road users.
The collision can be avoided automatically if the
difference in speed between your vehicle and the
vehicle ahead does not exceed 9 mph (15 km/h).
Above this threshold, the system will do what
is possible to avoid or reduce the accident by
lowering the speed of impact.
The automatic braking may bring the vehicle to
a complete stop if the situation requires it.
In this case, the vehicle is held stationary
temporarily (for about 1.5 seconds) to allow
the driver to take back control by pressing the
brake pedal.
The triggering of the system may cause
the engine to stall, unless the driver
depresses the clutch pedal quickly
enough during automatic braking.
During automatic braking, the driver
can always obtain a higher rate of
deceleration than that controlled by the
system, by pressing firmly on the brake
pedal.
After an impact, the system is
automatically put out of service: it no
longer operates.
You must go to a CITROËN dealer or a
qualified workshop to have the system
made operational again.