SAE J1978/ISO 15031-4 specifies a complementary set of functions to be provided by an OBD-II scan tool. These functions provide complete, efficient, and safe access to all regulated OBD (on-board diagnostic) services on any vehicle which is compliant with SAE J1978/ISO 15031-4. The SAE J1978 content of this document is intended to satisfy the requirements of an OBD-II scan tool as required by current U.S. on-board diagnostic (OBD) regulations. The ISO 15031-4 content of this document is intended to satisfy the requirements of OBD requirements in countries other than the U.S., and includes functionality not required or not allowed in the U.S. 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/ISO 15031-5.

SAE J1978/ISO 15031-4 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 to the OBD-II scan tool itself. Differences in the SAE J1978 and ISO 15031-4 requirements are highlighted by bold italic text in the technical requirements sections of this document. Notable examples are:

• Current U.S. OBD regulations will permit ISO 15765-4 as an allowable OBD serial data link at 500 kbps only. Only those provisions of ISO 15765-4 that pertain to 500 kbps are required to be supported by an SAE J1978 diagnostic scan tool.
• Current U.S. OBD regulations will not allow greater than 20 V at the SAE J1962 connector. Only the Type A connector as defined in SAE J1962/ISO 15031-3 needs to be supported by an SAE J1978 diagnostic scan tool.