The brief
The 316i's parking sensors had got unreliable, sometimes no beeps at all, sometimes erratic beeping with nothing behind, no visual display in some cases, a warning light on the dash, and a slow response. He brought it in to get the system working again. The parking sensors are the small ultrasonic sensors in the bumpers that bounce a signal off whatever's near and time the echo to work out how close it is. When one of them fails, it can't read properly, so the system gets a bad input, which is the missing or erratic beeps and the warning, and a faulty sensor doesn't recover. A parking aid you can't trust is worse than none, so it needs the bad sensor identified and changed.
The diagnosis
A scan showed the fault sitting with one parking sensor, its signal dropping out or reading implausibly, while the rest of the sensors and the module checked out. So it was that one sensor at fault. That's a sensor replacement. A failed parking sensor doesn't come back, so the sensor was getting changed and the system reset.
The work
The faulty parking sensor was removed from the bumper, and a new genuine BMW-spec sensor fitted in its place, the connector clicked back home and the bumper trim refitted. Then the stored fault was cleared and the parking system tested all round to confirm it was reading and beeping correctly. A check confirmed the sensors picking up obstacles cleanly, the right beeps, the display working, and no warning.
The outcome
The parking sensors reading correctly all round, the right beeps, the display working, and no dash warning. The 316i went home with the parking aid working as it should. A flaky parking sensor is the kind of thing that lulls you into not trusting it, so swapping the bad one and resetting the system put it right.