Case IH 3722 – Engine Control Unit (ECU): Lambda sensor: LSU sensor temperature Ri exceeds the maximum limit

The Engine Control Unit (ECU) monitors the internal resistance (Ri) of the Lambda (O2) sensor's Nernst cell to calculate its operating temperature. When the Ri value exceeds the maximum programmed threshold, it indicates that the sensor is registering an abnormally high temperature. This is typically caused by an internal failure of the sensor itself (such as a degraded heater circuit or Nernst cell) or, less commonly, actual extreme exhaust overheating.

• Farmall
• Farmall 85A
• Farmall 95A
• Farmall 105A
• Farmall 115A
• Farmall 115U Pro EP
• Maxxum
• Puma
• Optum
• Magnum
• SR175

1. 1

   Inspect the Lambda sensor wiring harness and connectors for melting, chafing, or corrosion. Ensure the wiring is routed away from extreme exhaust heat.

2. 2

   Check the ECU pins associated with the Lambda sensor (IA, IP, UN, VG) for continuity and ensure there are no shorts to the battery or ground.

3. 3

   Verify that there are no severe exhaust restrictions (such as a heavily clogged Diesel Particulate Filter or Diesel Oxidation Catalyst) that could be causing genuine exhaust overheating.

1. 1

   Preparation

   Turn off the engine and allow the exhaust system to cool down completely to prevent severe burns. Disconnect the negative battery terminal.

2. 2

   Access the Sensor

   Locate the Lambda sensor on the exhaust aftertreatment system. Depending on the model (e.g., Farmall 115A), you may need to loosen or remove the exhaust heat shield and bonnet support to gain access.

3. 3

   Removal

   Disconnect the sensor's electrical connector. Use a specialized O2 sensor socket to carefully unscrew the Lambda sensor from the exhaust pipe.

4. 4

   Installation

   Apply a small amount of high-temperature anti-seize compound to the threads of the new sensor (if it does not come pre-applied), being careful not to get any on the sensor tip. Thread the sensor in by hand to prevent cross-threading, then tighten to the OEM torque specification.

5. 5

   Reassembly

   Reconnect the electrical connector, secure the wiring harness, and reinstall any heat shields or supports that were removed. Reconnect the battery.

6. 6

   ECU Reset

   Connect a diagnostic Electronic Service Tool (EST). Clear the fault codes and run the "Reset ECU data (Lambda sensor)" configuration routine. This forces the ECU to learn the calibration parameters of the new sensor.