The brief
The Passat's ABS warning light had come on and stayed on, and the traction control had disabled itself in the process. The owner could feel the difference under harder braking and brought the car in to get the system back online.
The ABS reads how fast each wheel is turning from a small sensor at the hub. When one of those signals fails, the ABS can't trust its picture of the road, so it puts up the warning and steps back, and the traction control, which uses the same readings, drops out with it. A warning that comes on and stays on, rather than flickering, usually means the sensor has failed outright rather than just going marginal.
The diagnosis
On the scanner the codes pointed cleanly at one sensor, the left front wheel speed sensor, with a string of faults logged against it, from open circuit to mechanical malfunction. A meter check at the connector confirmed the sensor itself was the failed component, with the harness and the tone ring it reads off both fine.
That makes it a sensor replacement. A failed wheel speed sensor doesn't recover, so the sensor was getting changed.
The work
The car went up, the failed wheel speed sensor was unbolted from the hub and a new VAG-spec sensor fitted with a fresh O-ring, the harness clipped back into its proper run so nothing rubs. Then the stored faults were cleared from the brake module and the system checked over.
A road test with a series of braking events confirmed nothing came back.
The outcome
ABS warning off, traction control back online, and no codes after a full drive cycle.
The Passat went home with the braking electronics working properly again. The ABS and traction control are there for the moment you need them, so getting the failed sensor changed and the system back online put the safety net back where it should be.