Case IH 3939 – Engine Control Unit (ECU) / EDC (Electronic Diesel Control): ECU Temperature Sensor: Physical Signal ECU Temperature second sensor above maximum limit

The secondary internal temperature sensor located inside the Engine Control Unit (ECU) is reading a temperature that exceeds the maximum allowable threshold. This indicates the ECU is either physically overheating due to external environmental factors, or the internal temperature sensing circuit on the ECU's motherboard has failed.

• Tractors
• Combines
• Floaters

1. 1

   Carefully feel the exterior housing of the ECU for excessive heat and inspect the surrounding area for missing heat shields, exhaust leaks blowing on the module, or heavy debris buildup.

2. 2

   Connect the CNH Electronic Service Tool (EST) or EASY diagnostic tool and monitor live data for the ECU internal temperature. If it reads impossibly high on a cold engine, the internal sensor has failed.

3. 3

   Check for accompanying codes that indicate internal board failures, such as 3814 (SPI Error ECU temperature sensor) or 3820 (Over temperature error for H-bridge).

4. 4

   Inspect the main ground straps and wiring harness connectors at the ECU for poor grounds or electrical shorts that could cause excessive current draw and internal overheating.

1. 1

   Address External Heat Sources

   If the diagnostic checks revealed missing heat shields, exhaust leaks, or debris, repair these issues and clean the ECU housing to restore proper heat dissipation. Clear the code and test run the machine.

2. 2

   Prepare for ECU Replacement

   If internal failure is confirmed, turn off the machine and disconnect the negative battery cable.

3. 3

   Disconnect ECU Wiring

   Carefully release the locking levers on the main wiring harness connectors and unplug them from the ECU.

4. 4

   Remove the ECU

   Unbolt the ECU from its mounting bracket (typically mounted on the side of the engine block or firewall).

5. 5

   Install Replacement ECU

   Install the replacement ECU and torque the mounting bolts to spec.

6. 6

   Reconnect Wiring and Battery

   Reconnect the wiring harnesses, ensuring the locking levers seat fully. Reconnect the battery.

7. 7

   Program the New ECU

   A dealer or technician with the CNH EST software must flash the new ECU with the correct software payload (dataset) for your specific machine's PIN/serial number before the engine will start.