Massey Ferguson 4344.04 – Engine Control Unit (ECU): DEF Unit Internal Temperature Sensor Issue (ECU)

The sensor that monitors the temperature of the Diesel Exhaust Fluid (DEF / AdBlue) inside the tank is reporting an error (Voltage Below Normal or Shorted to Low Source). If the ECU loses this signal, it cannot trust the Selective Catalytic Reduction (SCR) system, which will trigger warning lights, disable DEF dosing, and eventually force the tractor into a severe engine derate (limp mode).

• 4710
• 7724S
• 5712SL
• Interim Tier 4 (2012-2015)
• 7722
• 7722S
• 7615
• 5613
• 5713SL
• 7720S
• 7716
• 7718
• T4
• 7715
• 5710SL
• 7726
• 7616
• 7618
• 5608
• 4708
• 7619
• 7626
• 7726S
• 7624
• 4707
• 4709
• 5711SL
• 7614
• 7718S
• 5610
• 7620
• 7719
• 7714
• 7716S
• T4 (2015-2021)
• 5612
• 7622
• 7720
• Tier 4 (2015-2021)
• 5611
• 7715S
• 5609
• 7724

1. 1

   Inspect the wiring harness and the main electrical connector at the top of the DEF tank header for rubbed wires, corrosion, or moisture inside the connector pins.

2. 2

   Use a diagnostic scan tool to read the live DEF tank temperature. If the reading shows an impossible extreme (e.g., -40°C or +200°C), the internal sensor is shorted or open.

3. 3

   Verify that the tractor's Ambient Air Temperature sensor is reading correctly, as a failed ambient air temp sensor can disable the DEF system and trigger erratic DEF temperature fault codes.

4. 4

   Check for DEF fluid leaking out of the header O-ring and crystallizing on the electrical connector, which can destroy the sensor's circuitry.

1. 1

   Preparation

   Turn off the tractor, disconnect the battery, and thoroughly clean the top of the DEF tank around the header assembly to prevent dirt from falling into the tank.

2. 2

   Disconnect Lines

   Carefully disconnect the DEF supply and return lines from the header. Note: These are heated lines with quick connectors and O-rings. Cap them immediately to prevent contamination and do not kink them.

3. 3

   Disconnect Electrical

   Unplug the CANbus/electrical connector from the DEF header.

4. 4

   Remove Header

   Unscrew the locking ring or retaining bolts holding the DEF header to the tank. Carefully pull the header assembly straight up and out of the tank.

5. 5

   Install New Part

   Insert the new DEF header assembly, ensuring the new rubber gasket/O-ring is seated perfectly to prevent dust and moisture ingress.

6. 6

   Reassembly

   Tighten the locking ring/bolts. Reconnect the heated DEF lines (ensure they click into place) and the electrical connector.

7. 7

   Clear Codes

   Reconnect the battery, turn the ignition on, and use a diagnostic tool to clear the 004344.04 code and reset the aftertreatment system.