- just had these replaced on the little Smart car - like the BMW, it's a known problem.
Mine had a small crack in the ring.In normal use this was not a problem.
When the weather was freezing ,the water in the crack froze and expanded and threw up the ABS light.
New rings were cheap (about £20...) but the shafts had to be released ,all the rust/crud cleaned off and new rings slid into place.
I understand that the rings used to be interference fit and had to be heated before installing.Now they are just large enough to slide on and are then secured with a little Araldite or similar.
Mercedes (Smart) want several hundred pounds for complete shafts and ABS rings.....