Rear parking sensors stopped working

[Inversed]

After a sprited drive a week ago I almost reversed into parking garage wall because the parking sensor didn't engage. I distinctly remember it working during the drive when I was making 3-point turn.
Usually there is a beep when you put it reverse, that is gone. Took car few times out since, still not working.

Any ideas? Car is 390 FE. Official service is 250km away, if possible I would love to fix it before I take car there next year.

Edited April 8 by Inversed

[Paul_D]

Do the reverse lights come on? (Just trying to narrow it down - does the car know it's in reverse?)

[Inversed]

Yes, reversing light works 

[Paul_D]

Looking at the wiring diagram, if the reverse lights are working then I would say either the buzzer or the reverse sensor module must be faulty. (Unless a single sensor failure causes the whole system to just remain silent) 

The same 12v supply is fed to both the reverse light and the sensor module when reverse is selected, so if the lights have power, so do the reversing sensors. The only other connections on the reversing module are the 4 sensor inputs, and the buzzer output.

[RoflOnMyWaffle]

FWIW - the parking sensors have always been shit, and thats being nice.

Mine would work fine all day, then freak out in an empty car park as if I was 3 feet away from driving into someting. I obviously stop, check mirrors, cant see, get out and check - nothing but air.
Get back in, try reverse again, car is fine.

It will randomly go off when I reverse down my driveway, to the point I ignore it now, because I know there is nothing there. It's not good.

[Inversed]

Just update on this issue:

I went on track this saturday and issue fixed itself. I guess the car just had abstinence crisis from track. Now I don't trust the system anymore, I hope Aim updates dash, so camera will be an option soon.