depends on the vehicle, on older vehicles (say late 80's back) it is usually a busted tooth on the adapter in your transmission or your cable is kinked, on new cars, it is all computerized and usually means that your cluster panel (where all your gauges, speedometer and odometer are located) is slowly going out. Usually cheaper to replace the whole cluster panel than it is to replace just that one component, but check with your dealership about prices in your area.