[magduf]

OBD-II isn't actually a real standard. The OEMs all tack on additional stuff to it, so none of the implementations are fully compatible with each other. Ford and Mazda, for instance, use a medium-speed CAN bus that other OEMs don't, so most OBD-II readers can't access anything on that bus. Also, the whole point of OBD was supposed to be standardizing diagnostic messages so anyone could read the trouble codes. However, all the OEMs have lots of proprietary codes that aren't in the standard. The whole thing is a mess.

Finally, OBD-II isn't a standard bus anyway. It's an interface to the end user or technician. The various modules on the car are interconnected with a variety of buses: CAN (high-speed or low-speed or medium-speed), LIN, MOST, etc.