The brief
The B200 had its ABS and ESP lights on, and a defective wheel speed sensor will do exactly that, light the ABS or stability warning, sometimes set a check engine light, and in rare cases drop the car into limp mode. When you get a warning like that the right move is a scan to confirm what's wrong, and that's what he came in for. The wheel speed sensors tell the ABS and stability module how fast each wheel is turning so they can step in if one locks or spins. They sit down by the hubs, exposed to heat, water and road grime, and they fail, the element going or the wiring corroding. When one stops giving a clean signal the module can't trust the data, so it shuts ABS and ESP down and lights the dash. A failed sensor doesn't recover, so it needs replacing.
The diagnosis
A diagnostic scan pulled the fault straight to a rear wheel speed sensor, no clean signal coming from it, which is exactly what trips the ABS and ESP lights. The other three sensors and the brakes themselves checked out, it was that one sensor. That's a sensor replacement on the affected rear corner, you don't repair a failed speed sensor, so the call was a new genuine sensor, fitted and the codes cleared.
The work
The rear wheel came off, the failed ABS sensor was removed from the hub, the mounting cleaned up, and a new genuine Mercedes-spec wheel speed sensor fitted and routed properly so the wiring's protected. The system was scanned to confirm a clean signal from all four wheels and the stored ABS and ESP codes cleared. A road test confirmed the ABS and ESP warning lights stayed off and the systems were active again.
The outcome
No ABS or ESP warning, the anti-lock and stability systems back online, no limp mode, and a clean signal from every wheel. The B200 went home with the safety systems working again. A failed wheel speed sensor takes ABS and ESP offline until it's fixed, so changing it on the affected rear corner and clearing the codes put the braking systems back where they should be.