Circuit DescriptionThe engine control module (ECM) monitors the NOx concentration in the engine 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 selective catalytic reduction (SCR) catalyst and dosing system.

Component LocationThe location of the diesel exhaust fluid (DEF) tank is original equipment manufacturer (OEM) dependent. The SCR catalyst and aftertreatment NOx sensors are located in the vehicle’s exhaust system.

Conditions for Running the DiagnosticsThis diagnostic runs after the DEF tank has been refilled. Specific engine and aftertreatment conditions must be met. 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, and the DEF tank has recently been refilled.

Action Taken When the Fault Code is ActiveThere are three levels of action taken by the ECM when this fault code is active:

Level 1: Action taken immediately after the diagnostic runs and fails.

The ECM illuminates the amber CHECK ENGINE light and/or the malfunction indicator lamp (MIL).Level 2: Action taken if the engine is operated for a limited period of time with the fault code active:

The ECM illuminates the amber CHECK ENGINE light and/or the MIL.Engine torque will be reduced.Level 3: Action taken after extended engine operation with the fault code active, after engine shutdown, or extended idle with fault code active.

The ECM illuminates the amber CHECK ENGINE light and/or the MIL.The ECM illuminates the red CHECK ENGINE light and/or the MIL.Engine torque will be severely reduced.Vehicle speed will be limited to 8 km [5 mi] per hour.Conditions for Clearing the Fault CodeWhen all the results of the Dynamic Engine System Analysis Test have passed, then repair has been validated.Note: This fault code will still be active.

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

The SCR control system is able to detect when anything other than pure DEF is in the DEF tank.

Possible causes of this fault include:

Malfunctioning DEF dosing systemExhaust system leaksMalfunctioning SCR catalystDegraded, diluted, or incorrect DEFDEF 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 DEF dosing unit filter failure2) ECM calibration