John Deere 3031.04 – Engine Control Unit (ECU): DEF Tank Fluid Temperature Sensor, Voltage Too Low

The Engine Control Unit (ECU) has detected a low voltage signal from the DEF tank temperature sensor, indicating the voltage is below the normal operating range. This sensor is critical for the Selective Catalytic Reduction (SCR) system to monitor DEF temperature and activate the DEF heater to thaw frozen fluid. An inaccurate reading can prevent the heater from activating, leading to reduced emissions control efficiency and potentially forcing the engine into a power-reduced limp mode to protect the aftertreatment system.

• 5085E
• 5090E
• 5090EL
• 5100E
• 5090M
• 5100M
• 5100MH
• 5100ML
• 5115M
• 6110M
• 6120M
• 6130M
• 6145M
• 6155M
• 6175M
• 6195M
• 6110R
• 6120R
• 6130R
• 6145R
• 6155R
• 6175R
• 6195R
• 6215R
• 330G
• 331G
• 332G
• 333G
• 6105M
• 9470RT
• 9520RT
• 9570RT
• R4023
• W235
• W260

1. 1

   Inspect the DEF tank temperature sensor (integrated into the DEF header) and its associated wiring harness for signs of physical damage, chafing, or loose contacts.

2. 2

   Disconnect the DEF header connector and the ECU connector to check for corrosion, moisture, or bent pins that could be causing a short to ground.

3. 3

   Measure the voltage at the temperature sensor circuit to confirm if it matches expected specifications (a lower-than-expected voltage confirms a short to ground).

4. 4

   Test the wiring harness for continuity between the sensor connector and the ECU to isolate whether the short is in the harness or the sensor itself.

1. 1

   Repair Wiring

   If the diagnostic tests reveal a short to ground in the wiring harness, repair or replace the damaged section of the harness.

2. 2

   Replace the Sensor/Header

   If the wiring and ECU outputs are intact, the fault lies within the sensor. Replace the entire DEF Tank Header Assembly, as the DEF temperature sensor is integrated and cannot be replaced individually.

3. 3

   Clear Codes and Reset

   After the repair is complete, reconnect all harnesses. Use a diagnostic scan tool to reset the ECU and clear the fault codes.

4. 4

   Inducement Unlock

   Because this fault causes an emissions inducement (engine derate), you must perform an 'Inducement Unlock' or 'Counter Reset' via the diagnostic software to restore full engine power.