FMI codes are defined in SAE J1939-73 Appendix A and describe the specific failure mode rather than which component failed. Each FMI is a 5-bit value ranging from 0 to 31, with standard definitions such as:
As of 2025, a Work-in-Progress (WIP) version of J1939-73 is being developed to add support for EPA27 engines and further enhance DM5 specifications.
This message is a request sent from a diagnostic tool to an ECU to clear its memory of previously active DTCs. After the root cause of a problem is fixed and verified, DM3 is used to erase the DM2 "history log" for that specific fault. It does not clear active faults; DM11 is used for that purpose. Sae J1939-73 Pdf
: Supports On-Board Diagnostics (OBD) requirements regulated by the EPA, EU, and California.
Several DM messages provide sophisticated diagnostic capabilities beyond basic fault reporting: FMI codes are defined in SAE J1939-73 Appendix
SAE J1939-73 standardizes how Electronic Control Units (ECUs) communicate diagnostic data to scan tools, fleet management systems, and data loggers. The protocol serves several vital functions across the vehicle lifecycle:
ECUs monitor each other. For example, a transmission ECU might monitor the engine ECU's DM1 message to determine if it should enter "Limp Home" mode. Off-Board: After the root cause of a problem is
Decades ago, if a semi-truck broke down, the mechanic was often blind to what was happening inside the engine's electronic control modules (ECUs). In the late 1990s, the introduced the J1939 series to replace aging, slower protocols. While other parts of the standard handled how the engine "thinks" or "moves," the J1939-73 sub-document was born specifically to handle Diagnostics . How the Story Unfolds (The Diagnostic Process)