Circuit DescriptionThe ECM monitors the NOx concentration in the engine’s exhaust gases using an aftertreatment intake NOx sensor and 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. Critically low NOx conversion efficiency is an indication of a missing SCR catalyst.

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. NOx sensor data is gathered over varying engine speeds and loads. When enough data has been gathered, the diagnostic will run, make a pass/fail decision, and begin to gather data again.

Conditions for Setting the Fault CodesThe ECM has detected that the NOx conversion efficiency of the SCR catalyst is critically low.

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 for two consecutive trips.Diesel exhaust fluid injection into the aftertreatment system is disabled.Engine torque will be reduced if the engine is operated for a limited period of time with the fault code active.Engine torque will be severely reduced and vehicle speed will be limited to 8 km [5 mi] per hour after extended engine operation with the fault code active.Conditions for Clearing the Fault CodeAfter completing a stationary regeneration (as instructed in this fault code troubleshooting tree) and no additional faults become active, the repair has been validated. Note that this fault code will still be activeIt 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 MIL

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

Possible causes of this fault include:

Incorrect calibration revision on INCAL? CDTampering with SCR catalyst system, removal of SCR catalyst from vehicle, or malfunctioning SCR catalystMalfunctioning diesel exhaust fluid dosing systemExhaust system leaksDegraded, diluted, or incorrect diesel exhaust fluidDiesel exhaust fluid deposits in decomposition tube.This fault code can be set as a result of multiple failure modes. All steps of the troubleshooting tree must be completed.

Reference the appropriate OEM wiring diagram when troubleshooting circuits that utilize wiring supplied by the OEM.

For intermittent power supply and datalink communication issues with Aftertreatment Components, it is highly recommended that the OEM Power Distribution Center fuses and relays be thoroughly checked for loose, missing or intermittent connections.

Possible Cause:1) Aftertreatment NOx sensor failure2) Aftertreatment DEF dosing unit Filter failure3) Aftertreatment SCR catalyst failure4) ECM calibration