John Deere 4334.04 – Engine Control Unit (ECU): DEF Dosing Unit Signal Out of Range Low / DEF System Pressure Sensor Voltage Low

The Engine Control Unit (ECU) or Aftertreatment Control Module (ACM) has detected that the voltage signal from the DEF pressure sensor is below the normal specified range. This indicates a potential short to ground, open circuit, or internal sensor failure. An incorrect low-voltage signal leads to inaccurate DEF pressure readings, causing improper DEF injection, impaired NOx reduction, and likely an engine derate to ensure emissions compliance.

• 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
• 5075M
• 5085M
• 9470RT
• 9520RT
• 9570RT

1. 1

   Inspect the wiring harness between the ECU/DCU and the DEF Dosing Unit (B5501) for damaged, frayed, or loose wiring that could cause a short to ground.

2. 2

   Test the supply, signal, and return pins at the DEF Dosing Unit connector (e.g., X5601 connector pins) to verify if the voltage is genuinely dropping below normal or if the harness is compromised.

3. 3

   Ensure there are no DEF leaks, crystallization, or contaminants around the dosing pump connector that could cause an electrical short.

4. 4

   Use a diagnostic tool (like John Deere Service ADVISOR) to run a DEF quantity/pressure test and monitor the DEF dosing unit components for proper operation.

1. 1

   Disconnect Power

   Turn off the machine and disconnect the battery to prevent electrical damage during the repair.

2. 2

   Address Wiring Issues

   If the diagnostic checks revealed a short to ground in the chassis harness, repair the damaged wires or replace the affected harness section.

3. 3

   Access the DEF Dosing Unit

   If the wiring is intact, the fault is likely internal to the DEF Dosing Unit (B5501). Locate the pump module on the machine.

4. 4

   Remove the Faulty Unit

   Disconnect the electrical connector and the DEF suction, backflow, and pressure lines from the dosing unit. Unbolt the unit from its mounting bracket.

5. 5

   Install the New Unit

   Mount the replacement DEF Dosing Unit. Reconnect the DEF lines securely to prevent leaks, and plug in the electrical connector.

6. 6

   Reassemble and Prime

   Reconnect the battery. Turn the key to the ON position to allow the system to prime.

7. 7

   Clear Codes and Verify

   Connect a diagnostic tool, clear the 004334.04 fault code from the ECU, and run a DEF dosing system test to verify that the pressure sensor is reading correctly and that there are no leaks.