Case IH 3725 – ECU / ECM: Lambda or Intake NOx Sensor Fault

Depending on the engine manufacturer, this fault code indicates either a short to ground in the Lambda (O2) sensor circuit (FPT engines) or an abnormal rate of change reading from the Aftertreatment 1 Intake NOx sensor due to soot or DEF contamination (Cummins engines). Both conditions affect the engine's emissions monitoring and aftertreatment systems.

• skid steer
• Magnum
• Puma
• Maxxum
• Farmall
• Steiger
• Quadtrac

1. 1

   Inspect the Lambda sensor wiring harness for visible chafing, melting, or a short to ground (FPT engines)

2. 2

   Check the Lambda sensor connector for water ingress, corrosion, or pushed pins (FPT engines)

3. 3

   Check for related active fault codes (such as 3748, 6459, or 6461) which indicate a Cummins NOx sensor issue

4. 4

   Remove the Intake NOx sensor and inspect the probe for heavy soot or contamination (Cummins engines)

1. 1

   Disconnect the battery (FPT engines)

   Disconnect the battery before beginning work on the FPT engine.

2. 2

   Locate sensor and repair wiring (FPT engines)

   Locate the Lambda sensor on the exhaust system (often near the pressure differential unit and temperature sensors). If wiring damage is found, repair the harness.

3. 3

   Replace Lambda sensor (FPT engines)

   If the wiring tests fine, the short is likely internal to the sensor. Remove the system heat shield if necessary, unscrew the old Lambda sensor, and install a new one.

4. 4

   Reconnect battery and clear codes (FPT engines)

   Reconnect the battery and use the CNH Electronic Service Tool (EST) to clear the fault code.

5. 5

   Review TSB before replacement (Cummins engines)

   According to Cummins Technical Service Bulletin (TSB240083), do not immediately replace the sensor.

6. 6

   Perform stationary regeneration (Cummins engines)

   Perform a full stationary (parked) regeneration to burn off the contamination on the sensor probe.

7. 7

   Operate engine under load (Cummins engines)

   Operate the engine under load to meet the 'Conditions for Clearing the Fault Code.'

8. 8

   Replace Intake NOx sensor (Cummins engines)

   If the fault code returns after the regeneration, the sensor has failed internally. Disconnect the NOx sensor module connector, unscrew the sensor from the exhaust pipe, and install the replacement. Clear the inactive codes.