The brief
The Passat had an ABS warning light coming on and off, the intermittent kind that's easy to dismiss until it's on for good. He brought it in to get it checked, which is the right call, the ABS is a safety system and a flashing warning means it's dropping out. 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, often intermittently at first. When one stops giving a clean signal the module can't trust the data, so it shuts ABS down. The reliable way to pin it down is a fault code scan, and here the fronts were at fault.
The diagnosis
A fault code scan pulled it to the front ABS speed sensors, the signals dropping out, which is exactly the intermittent warning. The rear sensors and the brakes themselves checked out, it was the front pair on their way out from the same mileage. That's a sensor replacement, you don't repair a failed speed sensor, and with both fronts at fault you do the pair, so the call was a new pair of genuine VW front ABS speed sensors.
The work
The front wheels came off, the failed ABS speed sensors removed from the hubs, the mountings cleaned up, and a new pair of genuine VW-spec front wheel speed sensors 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 Passat went home with the safety system working again. An intermittent sensor fault only gets worse until ABS is offline for good, so doing the front pair and clearing the codes put the braking system back where it should be.