It could be something as trivial as tyre pressure, or the vehicle being left for a period of time . You could try to activate the system by braking hard and see if that works . If that doesn't work why not purchase a diagnostic scanner ? ( someone will give you the link for a cheap one on E bay ) it will then give you a code, which in turn should lead to any fault which would cut down on garage bills . We have the scanner codes if you need them . ATB, Trevor