John Deere 5435.31 – Engine Control Unit (ECU): DEF Dosing Unit Pump Fault / DEF System Pressure Is Erratic

This diagnostic trouble code indicates that the supply pressure of the Diesel Exhaust Fluid (DEF) dosing unit pump is unstable or erratic. This instability prevents the system from accurately dosing DEF into the exhaust stream, which leads to emissions control issues. If left unaddressed, the machine will eventually be forced into a final power derate.

• 5085E
• 5090E
• 5090EL
• 5100E
• 6110M
• 6120M
• 6130M
• 6145M
• 6155M
• 6175M
• 6195M
• 6110R
• 6120R
• 6130R
• 6145R
• 6155R
• 6175R
• 6195R
• 6215R
• 5090M
• 5100M
• 5100MH
• 5100ML
• 5115M
• 6105M
• 9470RT
• 9520RT
• 9570RT
• 1010G Forwarders
• L-Series Loaders

1. 1

   Inspect the electrical connector at the DEF pump for DEF fluid ingress (a known issue causing internal circuit issues and erratic pressure readings)

2. 2

   Inspect wiring and harnesses associated with the DEF dosing unit pump for damage, fraying, or loose connections

3. 3

   Check DEF lines and filters for blockages, crystallization, or restrictions that might cause pressure instability

4. 4

   Verify DEF fluid quality to ensure it is not contaminated and review operating conditions for extreme temperature fluctuations

5. 5

   Test pump function using a John Deere diagnostic tool (Service ADVISOR) to run a DEF pump pressure test to verify if the pump can build and hold consistent pressure

1. 1

   Safe Shutdown

   Perform the proper machine shut down and battery disconnect procedure. Note: Failure to follow the proper battery disconnect procedure before servicing the DEF system can cause further ECU issues or repeat failures.

2. 2

   Access the DEF Pump

   Locate the DEF Dosing Unit Pump assembly (typically mounted near the DEF tank).

3. 3

   Clean/Inspect

   Disconnect the electrical connector. If the connector is full of DEF fluid, thoroughly clean the harness side of the connector.

4. 4

   Replace the Pump

   If DEF has entered the connector and caused an internal circuit issue (or if the pump failed the diagnostic pressure test), the DEF Dosing Unit must be replaced. Disconnect the DEF supply and return lines, unbolt the pump assembly, and install the new unit.

5. 5

   Reassembly

   Reconnect the DEF lines and the cleaned electrical connector. Reconnect the machine's battery.

6. 6

   Reset and Monitor

   Use a diagnostic tool to reset the ECU and clear the 005435.31 fault code. Run the machine and monitor the system to ensure stable pump pressure and normal operation.