Massey Ferguson E4334.04 – ECU: DEF System Pressure Sensor Voltage Low

The control module has detected that the electrical signal voltage from the Diesel Exhaust Fluid (DEF) pressure sensor has dropped below the normal operating range (typically below 0.1V). This is almost always caused by a short to ground in the sensor's wiring circuit, an open circuit, or an internal failure of the sensor itself.

• 7616
• 5612
• 7622
• Interim Tier 4 (2012-2015)
• 7614
• 5611
• 7615
• 5610
• 5613
• 7620
• 5609
• 7618
• 5608
• 7619
• 7626
• 7624

1. 1

   Check for multiple codes using a diagnostic scanner. If codes for '5V Sensor Supply Voltage Low' (e.g., SPN 9021/9022) are present, diagnose the 5V reference circuit first, as a shorted 5V supply will cause all sensors on that circuit to report low voltage.

2. 2

   Perform a visual inspection by turning the key ON (Engine OFF), disconnecting the DEF pressure sensor electrical connector, and inspecting the pins for corrosion, water intrusion, or white DEF crystallization which can bridge pins and cause a short.

3. 3

   Perform a wiring harness test with the sensor and ECU/ACM disconnected. Use a multimeter to measure the resistance between the DEF pressure sensor signal pin on the harness and a known good chassis ground. If resistance is less than 1k ohms, the wiring harness is shorted to ground.

4. 4

   Test the sensor itself. If the wiring harness tests good (open circuit to ground), the internal circuitry of the DEF pressure sensor has likely failed.

1. 1

   Power Down

   Turn off the tractor, remove the key, and disconnect the negative battery cable to prevent electrical spikes.

2. 2

   Locate the Sensor

   Locate the DEF supply pump module (usually mounted near the DEF tank). The DEF pressure sensor is plumbed into this module to measure the pressure of the fluid being sent to the dosing injector.

3. 3

   Clean the Area

   Thoroughly clean the area around the sensor/pump module. DEF systems are highly sensitive to contamination; dirt entering the system will destroy the DEF injector.

4. 4

   Harness Repair

   If your diagnostic checks revealed a short to ground, trace the harness back to the ECU, locate the rubbed wire (often where the harness routes along the frame), splice in a repair wire, and seal it with heat-shrink tubing.

5. 5

   Sensor Replacement

   Disconnect the electrical connector. Unthread the faulty DEF pressure sensor from the pump manifold. Install the new sensor with a new O-ring. Torque to OEM specifications. (Note: On some AGCO Power emissions systems, the pressure sensor is integrated into the DEF pump module and cannot be serviced separately. In this case, the entire DEF pump module must be replaced).

6. 6

   Reassembly & Clearing

   Reconnect the electrical harness and the battery. Use a diagnostic tool to clear the E4334.04 code.

7. 7

   Verification

   Start the engine and allow it to idle. Monitor the DEF pressure via live data to ensure it builds pressure correctly and the code does not return.