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J1939 Protocol Stack
J1939 Protocol Stack

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J1939 DTC

In SAE J1939, the acronym DTC stands for Diagnostic Trouble Code, also known as a fault code, and serves to identify the failed parameter. A DTC contains the Suspect Parameter Number (SPN) for the failed parameter, how many times failure has occured (OC), and how it has failed (FMI).
Active DTCs are transmitted by the DM1 message while non-active DTCs (i.e. historic) are transmitted by the DM2 message. The DM1 and DM2 messages may contain multiple DTCs which mean the message may be transmitted using the Transport Protocol (TP). Both the DM1 and DM2 messages are defined by the J1939-73 specification.

DTC Definition:

Suspect Parameter Number (SPN)19 bits
Failure Mode Identifier (FMI)5 bits
Occurrence Count (OC)7 bits
SPN Conversion Method (CM)1 bit
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