Saying "most cars use the CAN bus" is kind of like saying "the network uses Ethernet," though - the higher layer protocols are usually proprietary and one-off f

Saying "most cars use the CAN bus" is kind of like saying "the network uses Ethernet," though - the higher layer protocols are usually proprietary and one-off for a specific vehicle lineup. Even the standard diagnostic protocols like UDS rapidly become non-standard once you get to the "what's what" level. For example, $22 readLocalIdentifier is standardized as "read local identifier," but what each identifier means is again 100% proprietary. About the only thing that's completely standard is what's mandated by law: OBD-II required parameters and trouble codes. When it comes to trouble codes, even the set beyond the OBD-mandated basics are _also_ usually proprietary, requiring dealer tools or their clones to decode.