Cummins ISB CM850 (2003-06) Fault Code: 2771 PID: SPN: 3234 FMI: 9 Aftertreatment Outlet NOx Sensor – Abnormal Update Rate.

Posted by PerDiezel Solutions on January 18, 2026

Cummins ISB CM850 (2003-06) Fault Code: 2771 PID: SPN: 3234 FMI: 9 Aftertreatment Outlet NOx Sensor – Abnormal Update Rate. No communications or valid data transfer rate detected on the J1939 datalink between the ECM and the aftertreatment outlet NOx sensor.

On-Board Diagnostics (OBD) Information (Euro 4 Stage 1+ Certified Engines):

1)The ECM illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
2)An engine torque derate will be activated after 50 hours of engine operation with the fault code active.
3)The ECM turns OFF the malfunction indicator lamp (MIL) after 1 ignition cycle that the diagnostic runs and does not fail. The MIL lamp can not be cleared using the INSITE? electronic service tool.
4)The fault code will be cleared from memory after 400 days or 9600 hours of engine operation.

The NOx sensor is permanently attached to the NOx control module. They are serviced as a single component and can not be replaced individually.

Possible causes:

1)The aftertreatment outlet NOx sensor is not receiving power from the battery supply circuit in the OEM harness. This could be caused by an open circuit in the power supply wire, blown NOx sensor fuse, or low voltage supplied by the battery.
2)Open return circuit on the aftertreatment outlet NOx sensor circuit between the battery circuit aftertreatment outlet NOx sensor.
3)Open or short circuit on the J1939 datalink circuit between the engine ECM and aftertreatment outlet NOx sensor.
4)Intermittent communications between the aftertreatment outlet NOx sensor and the primary engine ECM on the J1939 datalink.
5)A damaged engine ECM
6)Incorrect engine calibration.

Possible repairs:

1)Replace the NOx? sensor.
2)Replace the aftertreatment outlet NOx sensor.
3)Tighten the loose connections, and clean the terminals.
4)Repair or replace the OEM harness
5)Repair or replace the terminating resistor in the OEM harness.
6)If necessary, calibrate the ECM.

Cummins ISB CM850 (2003-06) Fault Code: 2559 PID: SPN: 1845 FMI: 4 Transmission ECU to ECM Communication Lost – Abnormal Update Rate. No communication or an invalid data transfer rate has been detected between the ECM and the transmission ECU.

Cummins ISB CM850 (2003-06) Fault Code: 2558 PID: SID 57 SPN: 697 FMI: 4 Auxiliary PWM Driver #1 – Voltage Below Normal or Shorted to Low Source. Low signal voltage detected at the analog torque circuit.

Cummins ISB CM850 (2003-06) Fault Code: 2557 PID: SID 57 SPN: 697 FMI: 3 Auxiliary PWM Driver #1 – Voltage Above Normal or Shorted to High Source. High signal voltage detected at the analog torque circuit.

Cummins ISB CM850 (2003-06) Fault Code: 2556 PID: SID 70 SPN: 729 FMI: 4 Intake Air Heater #1 Circuit – Voltage Below Normal or Shorted to Low Source. Low voltage detected at the intake air heater signal circuit.

Cummins ISB CM850 (2003-06) Fault Code: 2555 PID: SID 70 SPN: 729 FMI: 3 Intake Air Heater #1 Circuit – Voltage Above Normal or Shorted to High Source. High voltage detected at the intake air heater signal circuit.

Cummins ISB CM850 (2003-06) Fault Code: 2554 PID: 95 SPN: 1209 FMI: 2 Exhaust pressure sensor circuit – data erratic, intermittent, or incorrect. An error in the exhaust pressure sensor signal was detected by the ECM.

WARNING!