Circuit DescriptionThe engine control module (ECM) monitors the mono-nitrogen oxides (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 selective catalystic reduction (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. 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 detects that the NOx conversion across the SCR catalyst is lower 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.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: The fault code will still be active.

It is necessary to use the “Reset All Faults” command in INSITE? electronic service tool to clear this fault code and turn off 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 code include:

Malfunctioning diesel exhaust fluid (DEF) dosing systemExhaust system leaksMalfunctioning SCR catalystDegraded, diluted, or incorrect DEFDiesel 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.

Possible Cause:1) Aftertreatment NOx Sensor failure2) Aftertreatment DEF dosing unit filter failure3) Aftertreatment DEF dosing valve failure4) ECM calibration