CITROËN DS4 Owner's Handbook Manual page 228

Front parking sensors
In addition to the rear parking sensors, the front
parking sensors are triggered when an obstacle
is detected in front and the speed of the vehicle
is still below 6 mph (10 km/h).
The front parking sensors are interrupted if
the vehicle stops for more than three seconds
in forward gear, if no further obstacles are
detected or when the speed of the vehicle
exceeds 6 mph (10 km/h).
The sound emitted by the speaker
(front or rear) indicates whether the
obstacle is in front or behind.
Deactivation/Activation of the
front and rear parking sensors
The function is deactivated by pressing this button.
The indicator lamp in the button comes on.
Pressing this button again reactivates the function.
The indicator lamp in the button goes off.
The function will be deactivated
automatically if a trailer is being towed
or a bicycle carrier is fitted (vehicle
fitted with a towbar or bicycle carrier
recommended by CITROËN).
Operating fault
In the event of a malfunction of
the system, when reverse gear
is engaged this warning lamp
is displayed in the instrument panel and/or a
message appears in the screen, accompanied
by an audible signal (short beep).
Contact a CITROËN dealer or a qualified
workshop.
In bad weather or in winter, ensure
that the sensors are not covered with
mud, ice or snow. When reverse gear is
engaged, an audible signal (long beep)
indicates that the sensors may be dirty.
When the vehicle is moving at a speed
below 6 mph (10 km/h), certain sound
sources (motorcycle, lorry, pneumatic
drill, etc.) may trigger the audible
signals of the parking sensor system.