Just thought of another thing you can try, when the problem exhibits itself:
Car with ignition on and in drive or reverse selected (but engine not running), to activate the front or rear sensors and make the issue happen. When the system is showing red to tell you it will not operate due to fault, then get outside the vehicle and go to the front or back, whichever shows red on the inside parking indicators, then either listen or put your finger lightly on each sensor. You should be able to hear the ticking noise of each sensor as it pulses or may feel it with lightly touching finger on the sensor.
This will allow you to do a basic check to see if one or more sensor is not ticking/pulsing like the rest.
You could also record phone video footage at the same time whilst checking each one and calling out the position before putting the phone camera close to it which should pick up the ticking noise. Can then be reviewed in quieter surroundings to check which one's mid at fault.
thanks
J
