how can i test them as i think one is broke as i hit a tow bar on a car that suck out along way and since then they have not worked sorry forgot to say it is a SEAT Alhambra 1.9tdi se
each sensor should make a clicking noise.the one that hit the towbar might be a good place to start.a scan could be done to find out which sensor is faulty.one sensor down screws the whole system
