J1939 Diagnostics: DM1, DM2 and the Structure of a DTC
Anyone reading out the fault memory of a commercial vehicle, an engine or an agricultural machine inevitably ends up with three abbreviations: SPN, FMI and OC. Together they form a Diagnostic Trouble Code (DTC), and it is transported by the diagnostic messages DM1 and DM2 defined in SAE J1939-73. This article explains how those two parameter groups are built, how to decompose a DTC correctly, and where hand-written parsers regularly get it wrong.