Case IH 3707 – Engine Control Unit (ECU): Lambda sensor: Open circuit at the lambda sensor Nernst cell pin.

The engine's control system has detected a break in the electrical circuit (an open circuit) specifically on the Nernst cell pin of the exhaust Lambda (Oxygen) sensor.

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1. 1

   Check the Lambda sensor visually to ensure it is not unplugged or physically damaged.

2. 2

   Inspect the 5-pin electrical connector at the Lambda sensor for loose, corroded, or backed-out pins.

3. 3

   Using a multimeter, check the wiring harness between the Lambda sensor connector and the Engine ECU for continuity, specifically looking for a broken wire (open circuit) on the Nernst cell signal line.

4. 4

   Verify there are no short circuits to ground or power in the sensor lines.

5. 5

   Measure the internal resistance (Ri) of the sensor's heater circuit to determine if the sensor has failed internally (often showing an open circuit).

1. 1

   Power Down

   Turn the battery cut-off switch to the OFF position and wait at least 1 minute. This allows the various engine control units to save data and disconnect properly.

2. 2

   Locate the Sensor

   Locate the Lambda sensor on the exhaust system (typically mounted near the DOC/DPF or exhaust gas recirculation area).

3. 3

   Clear Access

   If necessary, loosen or remove the exhaust system heat shields to gain clear access to the sensor.

4. 4

   Disconnect

   Unplug the electrical connector from the Lambda sensor.

5. 5

   Remove Sensor

   Using an appropriate O2 sensor socket or wrench, carefully unscrew the faulty Lambda sensor from the exhaust pipe.

6. 6

   Install New Sensor

   Apply a small amount of high-temperature anti-seize compound to the threads of the new sensor (if it doesn't come pre-applied) and thread it into the exhaust pipe. Tighten to the manufacturer's specified torque.

7. 7

   Reconnect

   Plug the electrical harness connector back in, ensuring it clicks securely into place.

8. 8

   Reassemble

   Reinstall any heat shields or brackets that were removed.

9. 9

   Power Up & Clear Codes

   Turn the battery cut-off switch back to the ON position. Connect the CNH EST (Electronic Service Tool) or EASy diagnostic tool to clear the 3707 fault code and perform any required self-learning parameter resets.