The brief
The GLA200 had a squealing noise on braking, the wear-indicator chirp that means the front pads have worn down to the warning point. He brought it in, which is the right call, that squeal is the warning that comes before the grind. The pads are the wear item, designed to be used up and replaced, and the discs they clamp wear too, thinning and scoring over their life. As the pads thin a metal tab starts to touch the disc and squeals to tell you it's time, and if it's left the material runs out and the backing plate grinds the disc. The GLA200's front pads had reached that point and the discs had reached the end with them, so the front needed pads and discs together.
The diagnosis
On the lift the front brakes confirmed it: the pads worn to the indicator with little material left, and the discs scored and below minimum thickness. The calipers and slides were freed off and checked, they were fine, just the pads and discs gone. The rears still had life. So it was a front axle set: new genuine Mercedes-spec pads and discs on both sides together, 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 Mercedes-spec set of discs and pads fitted, the calipers and slide pins cleaned and greased so they move freely, every fastener torqued to spec. The pads were bedded in properly so they'd grip evenly from the start. A road test confirmed quiet, even, progressive braking with a firm pedal and no pulling.
The outcome
Quiet brakes, a firm pedal, even bite, sharp progressive stopping, and the squeal gone. The GLA200 went home stopping properly again. Worn pads turn into scored discs the longer they're left, so doing the front pads and discs as an axle set with genuine parts put the braking back where it should be, the safety job you don't put off.
