Case IH 3951 – Engine Control Module (EDC / Engine ECU): Physical Range check high (Exhaust Gas Temperature 1 Sensor Signal Too High)

The Engine Control Unit (ECU) is receiving a voltage signal from the Exhaust Gas Temperature (EGT) 1 sensor that exceeds the maximum expected physical range. This typically points to a short to battery voltage, an open circuit in the sensor wiring, or an internally failed sensor.

• Farmall 100C
• Farmall 105A/C/U
• Farmall 110A/C/U
• Farmall 115A/C/U
• Farmall 120U
• Maxxum 120
• Maxxum 125
• Maxxum 130
• Maxxum 140
• Puma series
• Steiger models
• Magnum models
• SR175
• L220
• STM N Series Loaders

1. 1

   Inspect the wiring harness and connector for the Exhaust Gas Temperature 1 Sensor (upstream of DOC/DPF) for melted wires, chafing, corrosion, or loose pins.

2. 2

   Use a multimeter to check the sensor signal wire for a short to battery voltage or an open circuit between the sensor connector and the Engine ECU.

3. 3

   Measure the resistance of the EGT sensor itself. If it reads completely open (infinite resistance) or is shorted internally, the sensor has failed.

4. 4

   Verify that the ECU is providing the correct reference voltage and a solid ground to the sensor connector.

1. 1

   Turn off the machine, remove the key, and disconnect the negative battery cable.

   Ensure the machine is powered down and battery is disconnected before starting repairs.

2. 2

   Allow the exhaust system to cool completely.

   The EGT sensor is threaded directly into the hot exhaust piping (usually just before the DOC/DPF assembly).

3. 3

   Disconnect the electrical connector from the Exhaust Gas Temperature 1 Sensor.

   Unplug the sensor from the wiring harness.

4. 4

   Repair wiring or proceed to sensor replacement.

   If the diagnostic checks revealed damaged wiring, repair the harness using OEM-approved splicing methods. If the sensor is faulty, proceed to the next step.

5. 5

   Unthread the EGT sensor from the exhaust pipe.

   Using an appropriate wrench or specialized sensor socket, carefully unthread the EGT sensor. Penetrating oil may be required if the sensor is seized.

6. 6

   Apply high-temperature anti-seize to the threads of the new EGT sensor.

   Apply a small amount of high-temperature anti-seize to the threads (if not already pre-applied by the manufacturer). Do not get anti-seize on the sensor probe.

7. 7

   Thread the new sensor into the exhaust pipe.

   Install the new sensor and tighten to the OEM torque specification.

8. 8

   Reconnect the sensor's electrical connector.

   Ensure the connector clicks securely into place.

9. 9

   Reconnect the battery and clear the fault code.

   Reconnect the battery, turn the key on, and use a diagnostic scan tool to clear the 3951 fault code.

10. 10

    Start the engine and verify the repair.

    Start the engine and monitor the exhaust temperature data on the scan tool to verify the sensor is reading within the normal physical range.