Cummins ISX12 CM2250 (2010-12) 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 sensor is a smart device that 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 using 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 aftertreatment 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 selective catalytic reduction (SCR) catalyst.

Conditions for Running the Diagnostics
This diagnostic consists of two parts.

The first part of the diagnostic performs when the SCR outlet temperature at the aftertreatment outlet NOx sensor is above 200?C [392?F] and the engine is in a motoring condition.
The second part of the diagnostic performs when the SCR outlet temperature at the aftertreatment outlet NOx sensor is above 200?C [392?F] and the calculated NOx value is changing.
Conditions for Setting the Fault Codes
The ECM has detected that the aftertreatment outlet NOx sensor reading is not zero during motoring conditions for two consecutive trips or the ECM has detected the aftertreatment outlet NOx sensor reading is not changing with engine conditions for two consecutive trips.

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 on two consecutive trips.
Conditions for Clearing the Fault Code
To validate the repair, start and operate the engine to raise exhaust temperatures. This can be done by either driving the vehicle or initiating a stationary regeneration using INSITE? electronic service tool.
The SCR outlet temperature at the aftertreatment outlet NOx sensor must be above 200? C [392? F] before the sensor can perform its internal diagnostics.
The fault code status displayed by INSITE? electronic service tool will change to INACTIVE immediately after the diagnostic runs and passes.
The ECM will turn off the amber CHECK ENGINE lamp after the diagnostic runs and passes.
For On-Board Diagnostics (OBD) engines, the ECM will turn off the MIL after three consecutive trips where the diagnostic runs and passes.
The ?Reset All Faults? command in INSITE? electronic service tool can be used to clear active and inactive faults, as well as extinguish the MIL for OBD applications.

Shop Talk
Possible causes of this fault code include:

Malfunctioning aftertreatment outlet NOx sensor
Diesel exhaust fluid crystallization or deposits
Improperly installed aftertreatment outlet NOx sensor.
Reference the appropriate original equipment manufacturer (OEM) wiring diagram when troubleshooting circuits that utilize wiring supplied by the OEM.

For intermittent power supply and data link 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) ECM calibration
2) Aftertreatment outlet nox sensor failure
3) Aftertreatment DEF dosing Valve failure