Parking distance control

Hi all, I've got a problem with my rear parking sensors on my '04 Disco 2. When I switch them on, I get a continuous tone, and the LED on the switch stays on constantly. RAVE states that if a fault is detected that the system will give a constant tone for 3 seconds and the LED will flash, mine just gives a continuous tone (for as long as I am in reverse or switch the sensors on). I have replicated the fault condition by removing the sensor plug from the PDC ECU, so I know that the ECU is not detecting a fault. I have also listened to each of the sensors and they are all clicking indicating that they are working. It is like the ECU thinks that I am very close to something!!

Any ideas?

Cheers

(Any "do a search stuff" - yeah I did that already and didn't find anything)

Sorted by a good jet washing. Guess it must have been dust around the sensors.