The brief
The A1 had a warning light on the dash, the kind that comes on as you drive when a wheel speed sensor starts failing. He brought it in. The wheel speed sensor, also called the ABS sensor, monitors how fast each wheel is turning and feeds that to the car's computer. The ABS uses it to apply the right brake pressure when a wheel locks, the traction control uses it to catch a spinning wheel, and it feeds the speedometer too. When a sensor goes faulty, the computer can't trust that wheel's reading, so it shuts ABS and traction control 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 warning and deactivates ABS and traction control. 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 fault codes cleared. A road test confirmed the warning light stayed off and the ABS and traction control were active again.
The outcome
No warning light, ABS and traction control back online, and a clean signal from every wheel. The A1 went home with the safety systems working again. A failed wheel speed sensor takes ABS and traction control offline until it's fixed, so changing it on the affected corner and clearing the codes put the braking systems back where they should be.