Circuit DescriptionThe engine control module (ECM) monitors the NOx concentration in the engine’s exhaust gases using an aftertreatment intake NOx sensor and an aftertreatment outlet NOx sensor. 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 and decomposition tube.

Conditions for Running the DiagnosticsThis diagnostic has to meet specific engine and aftertreatment conditions to run and complete.

Conditions for Setting the Fault CodesThe Engine Control Module (ECM) detected the NOx conversion across the SCR catalyst was less than expected.

Action Taken When the Fault Code is ActiveThe ECM illuminates the amber CHECK ENGINE lamp and/or the Malfunction Indicator Lamp (MIL) immediately when the diagnostic runs and fails.Engine torque will be reduced if the engine is operated for an extended period of time with this fault active.Conditions for Clearing the Fault CodeIt is necessary to use the “Reset All Faults” command in INSITE™ electronic service tool to clear this fault and extinguish the amber CHECK ENGINE lamp and/or Malfunction Indicator Lamp (MIL).

Shop TalkTroubleshoot all other active NOx sensor and SCR dosing system related fault codes before this fault.

Possible causes of this fault code include:

Malfunctioning diesel exhaust fluid dosing systemExhaust system leaksMalfunctioning SCR catalystDegraded, diluted, or incorrect diesel exhaust fluidDiesel exhaust fluid deposits in the decomposition tube.For unit that has come back into with repeat failures consider the possibility that customer is not using Ultra-low sulfur Diesel fuel

Possible Cause:1) ECM calibration2) DEF fluid could be Contaminated3) ECM calibration