D2 rear parking sensor issue

The rear parking sensors on my D2 never made a noise when I first got it. So the first thing I did was replace the rear buzzer. It now makes noise when I select reverse, however it is a long tone and the light on the button on the dashboard simply flashes, and the parking sensors don't detect anything. I'm led to believe that this indicates there is a fault with the parking aid system.

The next thing I checked was the sensors themselves. I took each one out and checked the resistance between pins 1 and 2 and they all showed the same resistance. Which makes me think the sensors should be ok?

So on a whim I got a second control box to replace mine, but its still the same.

Is there anything else in the circuit that its likely to be? Was the way I checked the sensors good enough to detect a fault or is there another way? Any help would be appreciated

You're suposed to be able to hear the sensors clicking when you put your ear to them.
Griff