You open your and search for "SPN 91". You find:
Provides a history of faults that have occurred but are no longer active. DM3 (Diagnostic Data Clear): Used to reset or clear previously active fault history. DM11 (Diagnostic Data Clear/Reset for Active DTCs): Used to clear active faults from the ECU memory. DM12 (Emission-Related Active DTCs): Sae J1939-73 Pdf
Periodically broadcasts active Diagnostic Trouble Codes (DTCs) to the network, such as the instrument cluster, to inform the driver of the vehicle's status through lamp indicators. You open your and search for "SPN 91"