New Holland ENG 3721 – Engine Control Unit (ECU): Lambda sensor: Fault check to indicate SPI chip error of lambda sensor.

Indicates a communication or internal hardware failure involving the Serial Peripheral Interface (SPI) chip, which processes signals from the Lambda (Oxygen/O2) sensor. This usually points to a failed Lambda sensor, a severe wiring issue, or an internal failure of the engine control module itself.

• Boomer series
• T4 PowerStar
• T5.95
• T5.105
• T5.115
• T6 series
• T7 series
• T8 series
• T9 series
• W50C
• W60C
• W70C
• W80C
• L465
• LX465
• LX485
• LS180
• CR9090
• CX8080
• CX8090 Elevation

1. 1

   Visually inspect the Lambda sensor on the exhaust aftertreatment pipe and its wiring for heat damage, melting, or physical strikes.

2. 2

   Disconnect the Lambda sensor and inspect connector pins for corrosion, moisture, or pushed-out pins.

3. 3

   Trace the wiring harness back to the Engine ECU, looking for chafing, open circuits, or shorts to ground/battery.

4. 4

   Check for accompanying codes (e.g., 3724, 3725, or 3720). If ENG 3721 is the only code, the internal circuitry of the sensor is highly suspect.

5. 5

   Verify Engine ECU integrity if wiring is intact and a new sensor does not clear the code.

1. 1

   Turn off the machine's ignition and disconnect the negative battery cable to prevent electrical spikes.

   Ensure the machine is powered down before beginning electrical work.

2. 2

   Locate the Lambda sensor on the exhaust pipe and disconnect its electrical harness connector.

   The sensor is usually located near the DPF or SCR catalyst.

3. 3

   Carefully unscrew the sensor from the exhaust pipe using a specialized O2 sensor socket (typically 22mm or 7/8").

   If the sensor is seized due to exhaust heat, apply penetrating oil to the threads and allow it to soak before attempting removal.

4. 4

   Apply a small amount of high-temperature anti-seize compound to the threads of the new sensor.

   Most OEM sensors come with this pre-applied. Do not get any anti-seize on the sensor tip, as this will contaminate it.

5. 5

   Thread the new Lambda sensor into the exhaust pipe by hand to prevent cross-threading, then tighten it to the manufacturer's specified torque.

   Hand-threading ensures the threads are not damaged during installation.

6. 6

   Reconnect the electrical connector, ensuring the locking tab clicks into place.

   Secure the wire away from extreme heat sources.

7. 7

   Reconnect the battery.

   Restore power to the machine.

8. 8

   Connect the CNH Electronic Service Tool (EST) to clear the ENG 3721 fault code.

   Clear the code from the ECU memory.

9. 9

   Start the engine and allow it to reach operating temperature.

   Verify the code does not return and the exhaust system is functioning normally.