John Deere 4334.01 – ECU: DEF Dosing Unit Pressure Extremely Low

This diagnostic trouble code (DTC) is triggered when the Engine Control Unit detects that the pressure in the Diesel Exhaust Fluid (DEF) dosing unit is extremely low. Low pressure prevents the correct amount of DEF from being injected into the exhaust stream, which reduces the effectiveness of the Selective Catalytic Reduction (SCR) emission control system.

• 5085E
• 5090E
• 5090EL
• 5100E
• 5090M
• 5100M
• 5100MH
• 5100ML
• 5115M
• 6110M
• 6120M
• 6130M
• 6145M
• 6155M
• 6175M
• 6195M
• 6110R
• 6120R
• 6130R
• 6145R
• 6155R
• 6175R
• 6195R
• 6215R
• 9470RT
• 9520RT
• 9570RT
• 6105M
• 350G LC
• 9700
• 9800
• 9900

1. 1

   Verify DEF fluid quality using a refractometer to ensure it is ISO 22241-certified and at approximately 32.5% concentration.

2. 2

   Inspect the DEF dosing unit, supply lines, and dosing injector for physical leaks, kinks, or blockages caused by crystallized DEF.

3. 3

   Check the wiring harness and connectors leading to the DEF dosing unit and pressure sensor for fraying, damage, or poor grounding.

4. 4

   Verify that the DEF tank heaters and line heaters are functioning, as frozen DEF prevents the pump from building pressure.

5. 5

   Use a diagnostic tool to command the DEF dosing unit pump and monitor actual pressure output to confirm mechanical failure.

1. 1

   Resolve Fluid Issues

   If the DEF is contaminated or frozen, drain the tank, flush the system, and refill with fresh, high-quality DEF.

2. 2

   Clear Blockages

   If crystallized DEF is found in the lines or at the dosing injector nozzle, clean the components thoroughly with warm water. Replace the injector nozzle if it is permanently clogged.

3. 3

   Replace Filters

   Replace the DEF pump filter (often overlooked, which restricts fluid flow to the pump).

4. 4

   Replace the DEF Dosing Unit Pump

   If the lines are clear, the fluid is good, and the pump still fails to build pressure, the DEF dosing unit pump has failed and must be replaced.

5. 5

   Reset the ECU

   After repairs are made, use a John Deere diagnostic tool to clear the fault codes, reset the ECU, and run a forced regeneration or DEF dosing test to verify the system operates correctly and builds adequate pressure.