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

Conditions for Running the DiagnosticsThis diagnostic runs after the diesel exhaust fluid 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 diesel exhaust fluid 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 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 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 active

It 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.

The SCR control system is able to detect when anything other than pure diesel exhaust fluid (DEF) is in the diesel exhaust fluid tank.

Possible causes of this fault include:

Malfunctioning diesel exhaust fluid dosing systemExhaust system leaksMalfunctioning SCR catalystDegraded, 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.

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