Circuit DescriptionThe ECM monitors the NOx concentration in the engine’s exhaust gases, using aftertreatment intake NOx and aftertreatment outlet NOx sensors. By comparing these two values, the ECM is able to determine the conversion efficiency of the SCR catalyst and dosing system.

Component LocationThe SCR catalyst is located in the vehicle’s exhaust system, after the diesel particulate filter (DPF) and the decomposition tube.

Conditions for Running the DiagnosticsThis diagnostic only runs during INSITE? electronic service tool Aftertreatment Diesel Particulate Filter Regeneration Test.

Conditions for Setting the Fault CodesThe ECM detects that the NOx conversion across the SCR catalyst is lower than expected, indicating a malfunctioning SCR catalyst.

Action Taken When the Fault Code is ActiveThe ECM will log the fault code immediately when the diagnostic runs and fails.

Conditions for Clearing the Fault CodeUse the Reset All Faults command in INSITE? electronic service tool to clear active and inactive faults.

Shop TalkNOTE: This fault code is only used to assist in troubleshooting Fault Codes 3151, 3543, and/or 3582.

NOTE: Do not replace the SCR catalyst if Fault Code 3751 is the only active fault.

Possible Cause:1) ECM calibration