John Deere 4334.18 – ECU: DEF Dosing Unit Pressure Moderately Low

This code indicates that the pressure within the Diesel Exhaust Fluid (DEF) dosing unit has dropped below the expected operating range. Insufficient pressure prevents the proper atomization and injection of DEF into the exhaust stream, which reduces the effectiveness of the Selective Catalytic Reduction (SCR) aftertreatment system and can lead to engine derating (power reduction).

• 5085E
• 5090E
• 5090EL
• 5100E
• 5090M
• 5100M
• 5100MH
• 5100ML
• 5115M
• 6110M
• 6120M
• 6130M
• 6145M
• 6155M
• 6175M
• 6195M
• 6110R
• 6120R
• 6130R
• 6145R
• 6155R
• 6175R
• 6195R
• 6215R
• 5085M
• 6105M
• 7250R
• 7310R
• 8345R
• 8400R
• 9470RT
• 9520RT
• S680
• S690
• S760
• S790
• 330G
• 332G
• 331G
• 333G
• 350G LC

1. 1

   Verify DEF supply and quality to ensure the tank is adequately filled and fluid is not contaminated or crystallized.

2. 2

   Inspect DEF filters, as a restricted or plugged DEF pump filter is one of the most common causes of low pressure.

3. 3

   Inspect all DEF supply lines and the dosing unit for physical leaks, kinks, or blockages.

4. 4

   Perform a DEF pump pressure and high flow test using John Deere Service Advisor to verify if the pump can build and hold pressure.

5. 5

   Test pressure sensors to verify they are providing accurate readings and are not shorted or skewed.

1. 1

   Preparation

   Turn the battery disconnect switch to the OFF position before servicing the machine to isolate the electrical system.

2. 2

   Filter Replacement

   Locate the DEF dosing unit pump. Remove the filter housing cap, extract the old DEF filter element (e.g., RE554498), clean the housing, and install the new filter kit. Re-prime the system and test. If the code clears, the repair is complete.

3. 3

   Pump Replacement

   If the pump is worn out and fails the Service Advisor pressure test, it must be replaced.

4. 4

   Disconnect electrical harness

   Disconnect the electrical harness connectors from the DEF dosing unit pump.

5. 5

   Disconnect DEF lines

   Carefully disconnect the DEF supply and return lines. Note: DEF can be corrosive; clean up any spills immediately with warm water.

6. 6

   Replace pump

   Unbolt the old DEF dosing unit pump from its mounting bracket and install the new OEM pump.

7. 7

   Reconnect components

   Reconnect the DEF lines and electrical connectors securely.

8. 8

   Reset and Monitor

   Turn the battery disconnect back on. Connect John Deere Service Advisor, clear the 004334.18 fault code from the ECU, and run a forced system test or monitor the system during operation to ensure the new pump maintains the correct pressure.