Case IH 3904 – Engine Control Unit (ECU) / EDC: Fuel Temperature Sensor: failed plausibility check at ECU start

When the machine's ignition is turned on (ECU start), the engine computer runs a plausibility check to ensure sensor readings make logical sense before starting the engine. It compares the Fuel Temperature Sensor reading against other temperature sensors (such as the Coolant Temperature Sensor and Intake Manifold Air Temperature Sensor). On a cold start, these temperatures should be nearly identical. If the fuel temperature reading is drastically different from the others, the ECU determines the sensor's output is implausible and logs fault code 3904.

• CX series
• CS series
• CS Pro series
• C series
• Maxxum MX series
• MX100C
• MX90C
• Farmall series
• 115U
• CVX models

1. 1

   Visually inspect the wiring harness leading to the fuel temperature sensor for chafing, damage, or melting.

2. 2

   Disconnect the electrical connector at the sensor and inspect the pins for corrosion, moisture (diesel fuel or water intrusion), or loose terminals.

3. 3

   Using the CNH Electronic Service Tool (EST), turn the key to the ON position (engine off) and compare the Fuel Temperature, Coolant Temperature, and Intake Air Temperature. An impossible value (e.g., -40°C or 150°C on a cold engine) indicates a faulty sensor or circuit.

1. 1

   Locate the Sensor

   The fuel temperature sensor is typically located on the fuel filter housing assembly, the fuel rail, or threaded directly into the injection pump housing.

2. 2

   Prepare for Spills

   Place a rag or catch pan under the sensor area, as a small amount of diesel fuel will leak out when the sensor is removed.

3. 3

   Disconnect

   Unplug the electrical connector from the fuel temperature sensor.

4. 4

   Remove the Sensor

   Use a wrench to carefully unscrew the old sensor from its housing.

5. 5

   Clean the Port

   Wipe the threads and the mating surface on the housing clean to ensure a proper seal.

6. 6

   Install the New Sensor

   Thread the new fuel temperature sensor into the housing and tighten it securely.

7. 7

   Reconnect

   Plug the electrical connector back into the new sensor, ensuring it clicks into place.

8. 8

   Test

   Turn the key to the ON position to prime the fuel system and check for any leaks around the new sensor's threads. Clear the fault codes using a diagnostic tool and start the engine to verify the repair.