Cummins ISB4.5 CM2150 Fault Code: 3749 PID: SID 254 SPN: 3226 FMI: 20 Aftertreatment Outlet NOx Sensor – Data Not Rational – Drifted High.

Circuit Description
The aftertreatment outlet NOx (nitrogen oxides) sensor is a smart device and communicates with the engine control module (ECM) via the J1939 data link. The aftertreatment outlet NOx sensor performs its own internal diagnostics and reports malfunctions back to the primary ECM with the J1939 data link. The NOx sensor is permanently attached to the NOx control module. They are serviced as a single component and can not be replaced individually. The aftertreatment outlet NOx sensor is used to measure the NOx emissions at the outlet of the aftertreatreatment system.

Component Location
The Aftertreatment Outlet NOx Sensor location can vary, depending on the engine application. It is usually located in the exhaust system at the outlet of the aftertreatment SCR catalyst.

Conditions for Running the Diagnostics
This diagnostic runs when the engine is operating and the exhaust gas temperature at the aftertreatment intake NOx sensor is above 150° C [302° F].

Conditions for Setting the Fault Codes
The ECM has detected that the outlet NOx sensor reading is not changing to match the present engine operating conditions.

Action Taken When the Fault Code is Active
The 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 Code
-This fault code can not be cleared with INSITE™ electronic service tool. It is necessary to drive or operate the vehicle on a chassis dynamometer in order to validate the repair and for the fault code to go inactive.
-The exhaust gas temperature at the aftertreatment outlet NOx sensor must be above 150° C [302° F] before the sensor can run its internal diagnostics.
-The ECM will turn off the amber CHECK ENGINE light and/or MIL immediately after the diagnostic runs and passes.
-Use INSITE™ electronic service tool to verify that the fault code is inactive.
-The inactive fault code will be cleared from memory after 9600 hours of engine operation without a re-occurrence.

Possible Causes
-The aftertreatment outlet NOx sensor has been removed or tampered with
-Malfunctioning aftertreatment outlet NOx sensor.