i had the same problem. the PDC light was flashing green instead of solid (normal). when turned on it let out a long beep.

i took it to BMW and they said the flashing means an error and to book it in, which in australia means $250 an hour to diagnose, then your mother, grandmother and children to fix...

i took the advice here and turned ignition on for electrics only, tuned PDC on, put my ear to each of the 8 sensors and heard a clicking noise, sounds like an ignitor on a gas stove.

only one of the rear ones had no clicking noise, so i pressed it and it started to click faintly and the PDC started to work again, it beeped, as my head was on top of the sensor!

so it seems to be a bad rear sensor, it's working again now, could be a bad connection, which i will figure out how to replace or fix (about $50 for a used one, about $350 from BMW australia)

thanks BMW for telling me that it was likely a blown sensor!