Diagnostic messages in J1939 are defined in the SAE specification J1939-73 and can be send and received with the emotas J1939 Stack. Without the Diagnostic Messages extension the Diagnostic Messages (DM) need to be interpreted by the application.

The Diagnostic Messages extension provides API functions to send and receive Diagnostic Messages (DM1 .. DM64) in a more easy way.
One example are functions that combine Diagnostic Trouble Codes (DTC) from SPN, Failure Mode Indicator, Conversion Mode and Occurrence Count and map multiple of them into a DM that is transmitted by BAM or CMDT. Another example are special functions for the memory access and binary data transfer using DM14, DM15 and DM16.