How do you read a J1939 DTC?

A J1939 DTC contains four fields that relay information about the fault being reported by DTC. These fields are:

  1. Suspect Parameter Number (SPN): Represents the SPN with error. Every defined SPN can be used in a DTC.
  2. Failure Mode Identifier (FMI): Represents the nature and type of error that occurred, e.g., value range violation (high or low), sensor short-circuits, incorrect update rate, calibration error.
  3. Occurrence Counter (OC): A counter that counts the occurrence of the error condition for each SPN and stores this even when the error is no longer active.
  4. SPN Conversion Method (CM): Defines the byte alignment within the DTC. The value “0” represents the method shown in graphic “Structure of a DTC”. If CM has the value “1”, a distinction must be made between three previously valid methods; this must be known for the system.
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