John Deere 004334.11 – ECU: DEF System Pressure Did Not Drop During System Purge Cycle

The DEF system failed to properly relieve pressure after the machine was shut down. During a normal purge cycle, the system returns fluid to the tank to prevent crystallization and freezing. This fault indicates that pressure remained high, suggesting a blockage or malfunctioning component (such as a stuck pressure relief valve or failed pump) is preventing the pressure from being released. If unresolved, this can lead to an inoperative DEF system, engine power derates to ensure emissions compliance, and potential damage to DEF components due to sustained pressure.

• 7R Series
• 8R Series
• 8RT Series
• 8RX Series
• 9R Series
• 9RT Series
• 9RX Series
• 6R Series
• 6M Series
• 5E Series
• 5M Series
• 5R Series
• Self-Propelled Forage Harvesters
• L-Series Backhoes
• L-Series Loaders
• E-Series Skid Steers
• Various Construction & Forestry Equipment

1. 1

   Record and clear fault codes, then cycle the ignition and operate the machine to see if the 004334.11 code returns.

2. 2

   Check DEF quality. If the DEF is old or suspected of contamination, drain the tank and refill with fresh ISO 22241 standard DEF.

3. 3

   Visually inspect the DEF pump, lines, and injector for external leaks or white, crusty urea crystal buildup.

4. 4

   Inspect and clean or replace the DEF suction manifold filter and the DEF dosing unit filter, as clogs can restrict flow and affect pressure.

5. 5

   Inspect the DEF pump electrical connector for DEF fluid leakage, which indicates an internal seal failure requiring pump replacement.

6. 6

   Perform a DEF system pump test using a diagnostic scanner (like Service Advisor) to monitor if the system builds and releases pressure correctly during a purge cycle.

7. 7

   Examine the pressure relief valve (often integrated into the DEF pump/dosing unit) to ensure it is not stuck closed.

8. 8

   Check the DEF pressure sensor circuit voltage (power, ground, and signal wires) to ensure they are within the specified range.

1. 1

   Relieve system pressure

   Ensure any residual pressure in the DEF system is safely relieved before beginning work.

2. 2

   Disconnect components

   Disconnect the electrical connector and the DEF lines from the pump. Be prepared to catch any residual DEF fluid.

3. 3

   Remove the old pump

   Unbolt the DEF pump from its mounting bracket. On some machines, this may require removing access panels or dropping the DEF tank.

4. 4

   Install the new pump

   Mount the new DEF dosing unit pump to the bracket.

5. 5

   Reconnect lines and electricals

   Securely reconnect the DEF lines and the electrical connector to the new pump.

6. 6

   Prime the system

   Use a diagnostic tool to prime the DEF system and ensure all air is purged from the lines.

7. 7

   Clear codes and test

   Clear all fault codes and run the machine to verify that the repair has been successful and the code does not reappear.