Because D4P is designed in that way. It has no special meaning the rear speaker sounds when the vehicle is moving forward, as it might attract undue alerts. The objective of the sensor system to alert the driver of a possible collision with an obstacle when moving in one direction. If it does not move in that direction does not make sense alert.
Note that if the vehicle incorporates front and rear sensors when the vehicle moves , it is normal that the system only alerted acoustically front.
However if the vehicle is moved back, then the front speaker will sounds if detect a close obstacle, because in the backward movement the front area of the car can be approximated to a barrier to turn the vehicle.
In both cases, however the LED DISPLAY will keep to you informed about the distance to the obstacle and also by the vehicle OEM screen, if it is within the GROUP III compatibility. Check the behavior diagram manual for further information.
Note that in some cases some concepts of the logic of the sensor system manufacturer original D4P override. An example of this type occurs in BMW vehicles compatible with the GROUP III. In this case if there is an obstacle in the detection and rear vehicle moves forward the rear speaker sound. Since the speaker is controller by the radio’s own vehicle.
To extend information about this topic, please check these pages:
Posted in: 2. End user tech support and troubleshooting