Your ABS sensors send each wheels speed to the ABS controlling unit. If one of the sensors fail, a warning light will appear on your display alerting you of the situation. When an ABS sensor fails, the system is no longer able to figure out what the average speed of all four wheels is. Lets say you hit a slick patch of the road and your right wheel starts to spin. Ordinarily the ABS system would see an increase of speed in that one wheel and correct the situation. With even just one sensor out, it will not. This can be a dangerous situation if the driver is accustomed to it. Replace your worn out or broken sensors and make sure that everything is up to the task of snow, ice, and wet roads.