The brief
The A1 came in with two jobs: a light bulb out, and the ABS warning light on. Quick to diagnose, quick to fix, and worth doing both in one visit, a dead bulb is a safety and legal issue and ABS being offline is a safety one. The bulb is a wear item, it has a working life and it blows, and on a modern Audi it's worth fitting the correct spec. The ABS warning usually means a wheel speed sensor, the one at each wheel that tells the ABS module how fast each wheel is turning. 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 shuts ABS down and lights the dash. Neither gets better on its own, so it was a bulb replacement and a wheel speed sensor replacement.
The diagnosis
A check confirmed both: the blown bulb at the end of its life with the holder and wiring fine, and a diagnostic scan pulled the ABS fault to a wheel speed sensor on one corner, no clean signal coming from it. The other sensors and the brakes themselves checked out. So it was a bulb replacement, the correct spec, and a wheel speed sensor replacement on the affected corner, with the codes cleared.
The work
The blown bulb was replaced with the correct genuine spec, the holder checked. On the brakes, the affected wheel came off, the failed ABS sensor removed from the hub, the mounting cleaned up, and a new genuine Audi-spec wheel speed sensor fitted and routed properly. The system was scanned to confirm a clean signal from all four wheels and the stored ABS codes cleared, and the full lighting set run through. A road test confirmed the lighting working all round, the ABS warning off, and the anti-lock system active.
The outcome
The light working, all the lamps checked, no ABS warning, the anti-lock braking back online, and a clean signal from every wheel. The A1 went home with both sorted. A dead bulb and a failed wheel speed sensor are quick, cheap fixes that are safety issues if left, so doing both in one visit put the lighting and the ABS back where they should be.
