John Deere 003031.3 – ECU: DEF Tank Temp Sensor Voltage High

The Engine Control Unit (ECU) has detected a high voltage signal from the DEF Tank Temperature Sensor, indicating a potential open circuit, a short to a voltage source, or a sensor malfunction. The DEF temperature sensor is critical for the Selective Catalytic Reduction (SCR) system to maintain the DEF fluid within its effective temperature range. An incorrect high-voltage signal can trigger warning lights and cause the engine to enter a derated or limp mode to prevent damage and ensure emissions compliance.

• 8R 230
• 8R 250
• 8R 280
• 8R 310
• 8R 340
• 8R 370
• 8R 410
• 8RT 370
• 8RT 410
• 8RX 310
• 8RX 340
• 8RX 370
• 8RX 410
• 9R 390
• 9RX 640
• CTS Combines
• 135G
• 245G
• 345G LC

1. 1

   Use a diagnostic scan tool to confirm active fault code 003031.3 (SPN 3031 FMI 3) and note any related codes.

2. 2

   Visually inspect the DEF tank temperature sensor, connector, and wiring harness for corrosion (white, green, or blue discoloration), moisture ingress, bent pins, or chafed wires.

3. 3

   Check DEF level and verify DEF concentration with a refractometer, as low levels can sometimes contribute to high-temperature readings.

4. 4

   Disconnect the harness and test the wiring: check for a 5V reference voltage, verify ground circuit continuity to the chassis, and inspect the signal wire for shorts to a power source.

5. 5

   Test the sensor by measuring resistance across its pins and comparing to specs (e.g., 942 ohms at 0°C/32°F, 330 ohms at 25°C/77°F, 133 ohms at 50°C/122°F). Heat the sensor to ensure resistance changes smoothly.

1. 1

   Disconnect power

   Ensure the machine's ignition is off and disconnect the battery before starting any repair.

2. 2

   Access DEF tank header

   Locate the DEF tank and the header assembly on top of it.

3. 3

   Disconnect lines and connectors

   Carefully disconnect the main electrical harness connector, DEF supply, return, and coolant lines from the header fittings. Be prepared for residual fluid leakage.

4. 4

   Remove DEF tank header

   Remove the mounting bolts or locking ring securing the header to the tank, then carefully lift the assembly out.

5. 5

   Install new header or sensor

   Lubricate new O-rings with clean DEF. Insert the new header into the tank, align properly, and secure with mounting hardware tightened to manufacturer torque specifications.

6. 6

   Reconnect lines and connectors

   Reconnect the DEF lines, coolant lines, and the main electrical connector, ensuring all connections click securely into place.

7. 7

   Clear codes and verify

   Reconnect the battery, clear fault codes with a diagnostic tool, and run the engine to operating temperature. Monitor DEF tank temperature readings to ensure they are normal and the code does not return.