Imagine a massive tractor-trailer cruising down the highway. Suddenly, a sensor detects that the engine's coolant is running too hot. Deep inside the engine's "brain" (the ECU), the J1939-73 protocol springs into action. The DM1 Broadcast : The ECU doesn't just panic; it issues a Diagnostic Message 1 (DM1)
It standardizes how a vehicle reports a malfunction. A J1939 DTC consists of a Suspect Parameter Number (SPN), Failure Mode Identifier (FMI), Occurrence Count (OC), and SPN Conversion Method. sae j193973 pdf exclusive