I have the same problem on my 2009 Cali. I expect it is a faulty sensor, which is really easy to diagnose:
- switch ignition on (do not start engine)
- put van in reverse (so sensors come on)
- go to each sensor and put your ear to each in turn and you should hear a faint ticking sound
- if you don't, then it is likely that is your faulty sensor
One of mine is silent, causing the whole system not to work. I've not yet taken the sensor out - it is a rear one and it looks like I'll need a bit of work to get to the connection. Hints and tips beyond this point greatly appreciated...
Steve