The sensor is magnetic and sometimes can pick up some metallic fragments from the road that interrupt the Hall Effect sensor. Check that it's clean. It's the part that sits against the slotted ring on the left side of the wheel. Sometimes they fail and need replacing. Sometimes the connector under the motor gets damaged or water gets in.
There are front and rear sensors and the ABS system uses the info from both to decide when it needs to operate. When one is out of action the ABS won't work and the warning comes on.
Thank you. The dealership checked it out and said I need a new sensor. Sometimes abs works, speedometer doesn’t. Volume control reeves up. Blinkers don’t cancel. They tell me if I replace sensor everything should correct. Famous last words