Park assist problem

[BenV70D5]

Hi all,
When I put my 2008 v70 in reverse 2 of the parking sensors beep continuously as if an obstacle in right behind the car (solid blocks appear on screen too). The other 2 sensors function as normal.
The problem started out being intermittent and has become permanent.

Is it possible that 2 sensors have failed at the same time or would the likely culprit be the bumper wiring?

I've had the bumper off and cleaned the sensors, the wiring looks ok and everything was dry.. the problem didn't change when re-attached,

Thanks
Ben