The brief
The 318i had its brake warning light on, telling the owner the front brakes were worn below the minimum depth. He brought it in. That warning is the wear sensor doing its job, and the brake light coming on like that is the same message, the pads and discs are at the end. BMW runs a wear sensor in a pad on each axle that completes a circuit when the pad's worn down, lighting the dash. The discs the pads clamp wear too, thinning and scoring over their life. With the fronts worn below minimum the discs had reached the end with them, so it wasn't just pads, it was front pads, discs and a fresh wear sensor as an axle set.
The diagnosis
On the lift the front brakes confirmed it: the pads worn below minimum with the wear sensor ground through, and the discs scored and below minimum thickness. The calipers and slides were freed off and checked, they were fine. The rears still had life. So it was a front axle set: new genuine BMW-spec pads and discs on both sides with a new wear sensor, because you don't fit fresh pads to a scored, undersized disc, and you do the pair so both sides bite the same.
The work
Both front discs and the worn pads were removed, the hubs cleaned up, and a new genuine BMW-spec set of front discs and pads fitted with a new wear sensor, the calipers and slide pins cleaned and greased, every fastener torqued to spec. The brake warning was reset and the pads bedded in so they'd grip evenly from the start. A road test confirmed quiet, even, progressive braking with a firm pedal, the warning light off.
The outcome
Quiet brakes, a firm pedal, even bite, sharp progressive stopping, and the warning gone. The 318i went home stopping properly again. Pads worn below minimum turn into scored discs the longer they're left, so doing the front pads, discs and sensor as an axle set put the braking back where it should be, the safety job you don't put off.
