Case IH 3721 – Engine Control Unit (ECU / EDC): Lambda sensor: Fault check to indicate SPI chip error of lambda sensor

This code indicates a Serial Peripheral Interface (SPI) communication failure. The Engine Control Unit (ECU) is unable to communicate properly with the internal chip of the lambda (oxygen) sensor. This is an internal electronic fault either within the sensor itself or the engine controller's communication circuit.

• Farmall
• Farmall 115U Pro EP
• Puma
• Puma 210
• Magnum
• Steiger
• TR270

1. 1

   Inspect the wiring harness between the Lambda sensor and the Engine ECU for chafing, breaks, or melted wires

2. 2

   Inspect the sensor connector for corrosion, moisture, or bent pins that could interrupt the SPI data signal

3. 3

   Ensure the sensor is receiving proper battery voltage and ground

4. 4

   If wiring and voltage are correct, isolate the component as the internal SPI chip inside the lambda sensor has likely failed and the sensor must be replaced. In rare cases, it could be an internal communication chip error within the Engine Controller itself, requiring ECU replacement.

1. 1

   Preparation

   Ensure the machine is turned off and the exhaust system has completely cooled down, as the lambda sensor is located on the exhaust stream before the catalytic converter.

2. 2

   Removal

   Disconnect the 5-wire electrical connector for the lambda sensor. Use an oxygen sensor socket to carefully unthread the faulty lambda sensor from the exhaust pipe.

3. 3

   Installation

   Apply a small amount of anti-seize compound to the threads of the new sensor (if not pre-applied by the manufacturer). Thread the new sensor into the exhaust and tighten to OEM torque specifications. Reconnect the electrical harness.

4. 4

   ECU Reset

   Connect the CNH Electronic Service Tool (EST) to the machine's diagnostic port. Navigate to the engine configuration menu and select 'Replacement of the Lambda Sensor – Reset ECU Data'. This procedure restores the self-learning parameters for the new sensor.

5. 5

   Verification

   Start the engine and verify that fault code 3721 changes from 'Active' to 'Inactive' or clears entirely.