John Deere 4334.11 – Engine Control Unit (ECU): DEF Dosing Unit Pressure Signal Fault

The DEF dosing unit (DEF Supply Module) failed to relieve system pressure during a shut-off sequence or a new suction sequence. This indicates a failure in the pressure relief mechanism, a blockage in the return line, or a faulty pressure sensor.

• 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
• 8R 340
• 9470RT
• 9520RT
• 9570RT
• 9100
• 350G LC
• 210L
• 310L
• 344L
• 4630

1. 1

   Inspect DEF lines for physical blockages, kinks, or restrictions, paying special attention to the return line.

2. 2

   Monitor DEF dosing outlet pressure with the key on using a diagnostic tool or manual gauge.

3. 3

   Disconnect the DEF dosing return line (typically the middle line) and check if pressure is greater than 11 PSI during the purge cycle. If pressure spikes to 70+ PSI, the dosing unit is failing to relieve pressure.

4. 4

   Inspect the DEF fluid for dirt or crystallization that could be clogging the pressure relief valves.

5. 5

   Verify that the pressure sensors are providing accurate readings and are not shorted out.

1. 1

   Access the DEF Dosing Unit

   Locate the DEF supply module/dosing unit pump on the machine (often mounted near the DEF tank or aftertreatment system).

2. 2

   Relieve Pressure & Disconnect

   Carefully disconnect the DEF supply, return, and dosing lines. Note: Be prepared for fluid spillage if the system is still pressurized.

3. 3

   Remove the Dosing Unit

   Disconnect the electrical wiring harness (CANBUS and power plugs) and unbolt the DEF dosing unit from its mounting bracket.

4. 4

   Replace the Unit

   If diagnostics proved the internal relief mechanism failed (pressure wouldn't drop), install the new DEF Dosing Unit Pump.

5. 5

   Replace the Filter

   Replace the DEF Supply Module Filter (Filter Kit) while the system is open to prevent old contaminants from ruining the new pump.

6. 6

   Reassemble

   Reconnect all fluid lines and electrical harnesses.

7. 7

   Reset ECU

   Turn the key on, use a diagnostic scanner to clear the ECU 004334.11 fault code, and run the machine through a cycle to ensure the system pressurizes and purges normally without throwing the code again.