The brief
The A1 had its ABS warning light on, picked up on a maintenance check, the kind of thing early detection catches before it becomes a roadside problem. He had it sorted. The usual cause is a wheel speed sensor. There's one at each wheel, and they tell the ABS module how fast each wheel is turning so it can step in if one locks under hard braking. The sensors 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.
The diagnosis
A diagnostic scan pulled the fault straight to a wheel speed sensor on one corner, no clean signal coming from it, which is exactly 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 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 wheel came off, the failed ABS sensor was removed from the hub, the mounting cleaned up, and a new genuine Audi-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, and a clean signal from every wheel. The A1 went home with the safety system working again. A failed wheel speed sensor takes ABS offline until it's fixed, so changing it on the affected corner and clearing the codes put the braking system back where it should be.