More parking aid problems

Hi all, I’m new here, and have a problem with my parking aid which I hope someone can help with. I’ve scoured the forums and elsewhere online and can’t seem to find answers.

I have a 2010 Fiesta with factory-fitted front and rear parking aid. Recently, whenever I engage reverse or press the parking aid switch on the dash, the parking aid fails to work, I get a constant 3-4 second tone followed by a single beep (even while stationary), and the LED in the switch then flashes intermittently. The only way to stop it is to turn the ignition on and off again.

I’ve had a look myself and found the following:

With the ignition key in position 2 and either reverse engaged or the dash switch pressed, all sensors except the front left one are clicking normally. I have taken the front left sensor out and plugged it into the connector for the front right sensor and then it also clicks normally. Conversely, when plugging any of the other sensors into the connector for the front left they don’t click. I guess this implies the problem is in the wiring to the front left sensor, or the control module in the boot, rather than the sensor itself. I’ve checked the wiring as far as I can see it and also the control module and can see no obvious damage or corrosion. I guess the next step is to get a multimeter. By the way, all connectors have 3 wires, which I think means they are xvision ones?  

I have 3 questions:

1) Does anybody know what voltages I should be getting at the end connectors, and at the control module?

2) If all else fails, how do I simply disconnect the front sensors so that only the rear ones work?

3) Any other ideas??

Thanks all..

Rob