ECUs can self-report diagnostic information and self-compensate based on the data they receive. For example, an ECU might broadcast a DTC indicating a sensor out of range, which another ECU might use to adjust its operation.
Contains the Suspect Parameter Number (SPN), Failure Mode Identifier (FMI), and Occurrence Count (OC) for every active fault.
J1939/73_201901 : Application Layer - Diagnostics - SAE International Sae J1939-73 Pdf
(PGN 65226) DM1 messages are broadcast by an ECU when a fault is currently active. These messages are sent periodically—typically once per second—and provide real-time information about the vehicle's health. A DM1 message contains:
Without it, troubleshooting a modern truck would be a nightmare of proprietary connectors and undocumented blink codes. With it, a technician with a $200 J1939 adapter can instantly pinpoint a failing NOx sensor or a shorted wheel speed circuit. J1939/73_201901 : Application Layer - Diagnostics - SAE
This is arguably the most widely used diagnostic message. It is by any ECU that has an active (currently occurring) fault. This allows a diagnostic tool to see real-time, active issues without sending a request. The DM1 message contains the DTCs, along with an indicator for the Malfunction Indicator Lamp (MIL—the check engine light) and the number of active DTCs. In modern vehicles, DM1 is the core message for active fault broadcasting.
DM3: Diagnostic Data Clear/Reset for Previously Active Codes (PGN 65228) With it, a technician with a $200 J1939
Within the J1939 framework, . While J1939-71 covers general vehicle application layer parameters (engine speed, oil pressure, coolant temperature, etc.), J1939-73 focuses exclusively on diagnostic services, diagnostic messages (DMs), and the physical diagnostic connector interface.
J1939-73 provides methods for retrieving calibration identification numbers (CAL ID) and Calibration Verification Numbers (CVN). This is essential for regulatory bodies (like the EPA and CARB) to verify that the engine software is legitimate and hasn't been tampered with to bypass emissions standards.
For fleet managers, mechanics, and OEMs, J1939-73 delivers several benefits: