Because ODB-II codes are standardized, vehicle manufacturer's don't have to publish every code, but manufacturers typically publish model- or manufacturer-specific codes for mechanics. Because ODB-II codes were designed for mechanics and not drivers, manufacturers generally don't list them in owner's manuals.
Mechanics use a special tool to connect to the vehicle's computer system and enter a request. After the computer returns the code, the connected tool displays it on a screen. Because the ODB-II system is standardized across manufacturers, mechanics often memorize common codes over time. However, manufacturers can include their own codes; these start with "P1" and typically require a repair manual to interpret.