SAE J1939-73 Diagnostics Application Layer defines the SAE J1939 messages to accomplish diagnostic services and identifies the diagnostic connector to be used for the vehicle service tool interface. Diagnostic messages (DMs) provide the utility needed when the vehicle is being repaired. Diagnostic messages are also used during vehicle operation by the networked electronic control modules to allow them to report diagnostic information and self-compensate as appropriate, based on information received. Diagnostic messages include services such as periodically broadcasting active diagnostic trouble codes, identifying operator diagnostic lamp status, reading or clearing diagnostic trouble codes, reading or writing control module memory, providing a security function, stopping/starting message broadcasts, reporting diagnostic readiness, monitoring engine parametric data, etc. California, EPA, or EU regulated OBD requirements are satisfied with a subset of the specified connector and the defined messages. Replaced approximately 54 case where a cross reference was lost in publication process resulting in reference of “See 0”. Replaced many other lost links found during review. Fixed some confusing formatting.