The brief
The Polo's ABS warning had been coming and going, on for a drive and then clearing, then back again. Twice the traction control had switched itself off under harder braking. That on-off pattern usually means a sensor with a marginal connection rather than one that's failed outright.
The ABS reads how fast each wheel is turning from a small sensor at the hub. If one of those signals goes flaky, 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 sensor signal that flickers in and out is exactly what makes the warning come and go like this.
The diagnosis
On the scanner the faults sat in the brake module, and they were on the rear wheel speed sensors, one logging a mechanical malfunction and the other a range and performance fault. A meter check at the connectors showed the readings going in and out of spec when the wiring was flexed by hand, so the sensors themselves were the weak link, not the harness.
That makes it a sensor replacement. A flaky wheel speed sensor only gets worse, and you don't want the ABS dropping out mid-stop, so the affected rear sensors were getting changed.
The work
The car went up, the rear wheel speed sensors were unbolted from the hubs and new VAG-spec sensors fitted with fresh O-rings, 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 returning after a full drive cycle and plenty of braking.
The Polo went home with the braking electronics working properly again. A wheel speed sensor that flickers is the kind of fault that's easy to ignore until the ABS quits at the wrong moment, so sorting it now put the safety systems back where they should be.