SAE J1978-2 specifies a complementary set of functions to be provided by an OBD-II scan tool. These functions provide complete, efficient access to all regulated OBD services on any vehicle that is compliant with SAE J1979-2 and SAE J1979-3

The content of this document is intended to satisfy the requirements of an OBD-II scan tool as required by current U.S. OBD regulations.

This document specifies:

A means of establishing communications between an OBD-equipped vehicle and an OBD-II scan tool.
A set of diagnostic services to be provided by an OBD-II scan tool in order to exercise the services defined in SAE J1979-2.
The presentation of the SAE J1978 document family, where SAE J1978-2 covers second generation protocol functionality defined in SAE J1979-2, and SAE J1978-1 covers first generation protocol functionality defined in SAE J1979 and protocol determination for both SAE J1979 and SAE J1979-2.

The SAE J1978 document family does not preclude the inclusion of additional capabilities or functions in an OBD-II scan tool. However, it is the responsibility of the OBD-II scan tool designer to ensure that no such capability or function can adversely affect either an OBD-equipped vehicle, which may be connected to the OBD-II scan tool, or an OBD-II scan tool itself.

Differences in the SAE J1978-2 requirements are highlighted by bold italic text in the technical requirements sections of this document.