The brief
The C180 came in for its interim service, the owner keeping it on schedule, and the diagnostic scan that goes with it picked up an ABS fault and the warning light from a failing front wheel speed sensor. He had it sorted while it was in. The wheel speed sensors tell the ABS module how fast each wheel is turning so it can step in if one locks under hard braking. 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 down and lights the dash. A failed sensor doesn't recover, so it needs replacing, exactly the kind of thing the service scan is there to catch.
The diagnosis
The service scan pulled the fault to a front wheel speed sensor, no clean signal coming from it, which is what trips the ABS light. The other three sensors and the brakes themselves checked out, it was that one sensor. That's a sensor replacement on the affected front corner, you don't repair a failed speed sensor, so the call was a new genuine sensor, fitted and the codes cleared, alongside the interim service work.
The work
The interim service items were done, and on the brakes the front wheel came off, the failed ABS sensor 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 codes cleared. A road test confirmed the ABS warning light stayed off and the anti-lock system was active.
The outcome
No ABS warning, the anti-lock braking back online, a clean signal from every wheel, and the interim service done alongside it. The C180 went home with the safety system working again. A failed wheel speed sensor takes ABS offline until it's fixed, so catching it on the service scan and changing it on the affected corner put the braking system back where it should be.