Hi, i had sensor's fitted to mine at pdi by the dealer, after a short while they started to beep whenever i put it in reverse, but only during the day, after four unsucsessful visit's to the dealer i realised that it was only when it was hot i had the problem, when they fitted the sensor's they never rustproofed the hole's or/and made them slightly to small, so when it was hot the bumper expanded closing the hole on the sensor's or/and the hole's had rusted clamping the sensor's to tightly,so they beeped continually when in reverse,the sensor needs to vibrate to send a signal, so i removed the bumper, took out the sensors, enlarged all the holes by approx 1mm and rustproofed then painted them, refitted sensor's, bumper etc and never been a problem since, that was over two years ago, so the genuine sensors do work with metal bumpers if they are fitted correctly which it seems many dealers cannot manage to do, to me (as a technician with 32 years experience) its common sense to rustproof etc a metal bumper if you cut or the inevitable will happen, by the way the sensors are mounted differently to the ones fitted to plastic bumpers, so they are designed for metal bumpers, as i said they just need fitting properly. hope this helps Geoff.