Introduction
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The J1939 diagnostic interface defines a standard diagnostic connector as well as a set of PGNs for handling different diagnostic services. The PGNs designated as a Diagnostic Message (DM) largely fulfill the scope of functions of the UDS diagnostics (Unified Diagnostic Service). They also comply with the EU Directives and with the “California Code of Regulation” for Onboard Diagnostics (OBD II), as well as HD OBD (Heavy Duty OBD) and WWH (World Wide Harmonized) OBD (ISO 27145).

In contrast to the UDS diagnostics in which services have to be actively initiated via a software tool, J1939 ECUs also send diagnostic messages independently during standard operation. Errors that occur are evaluated directly in the network and visually displayed, if necessary. In parallel with that, errors can be read out with a tool via the diagnostic connector. Uniform error codes – so-called Diagnostic Trouble Codes (DTC) – contain the faulty SPN, the error pattern, and the frequency of occurrence of an error.