[henocsr]

As above, if you select reverse and get a long beep of about 10 sec, it is a warning that a fault is detected in the system and the system will not work. For the rest of the time the ignition is on the system will not work when every reverse is selected. When the car is keyed off and then back on again the system does a new check when reverse is selected for the first time, and if a fault is detected again, the long beep is sounded, etc etc.

What you can do it is put the car in reverse with the ignition on, but engine off. Got a listen to each of the reverse sensors. They should click continuously. The ones that don't are the ones that have caused the fault. So check the wiring or swap the sensors with the ones that do click and see if the fault moves. that way you can work out if the sensor is faulty, or the wiring is faulty