John Deere 4343.05 – ECU: DEF Dosing Unit Line Heater Circuit Has High Resistance

The control module has detected electrical resistance in the DEF line heater circuit that is too high, which typically indicates an open circuit (such as a broken wire, disconnected plug, or a burnt-out internal heater element). Because the heater cannot function properly, the DEF fluid is at risk of freezing in cold conditions, which will prevent DEF injection and eventually cause the engine to derate.

• 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
• Vermeer D24x40 S3
• Vermeer D40x55 S3

1. 1

   Inspect the wiring harness, connectors, and pins leading to the DEF dosing unit line heater for damaged, frayed, or loose wiring that could cause an open circuit.

2. 2

   Disconnect the DEF line heater connector and use a multimeter to test the resistance (Ohms) across the heater element pins. Infinite resistance (OL) or a value significantly above OEM spec indicates the internal heating element has failed.

3. 3

   Check the chassis-side harness to ensure the ECU is providing the correct voltage to the heater circuit when commanded on.

1. 1

   Turn off the machine and disconnect the battery to ensure the ECU is powered down.

2. 2

   Locate the faulty DEF line (Supply or Return) running between the DEF tank header and the DEF dosing unit/pump.

3. 3

   Disconnect the electrical connector for the specific DEF line heater.

4. 4

   Thoroughly clean the area around the fluid fittings to prevent dirt from entering the DEF system.

5. 5

   Disconnect the DEF line fluid fittings from both the DEF tank header and the dosing unit.

6. 6

   Remove the faulty DEF line from its routing clips.

7. 7

   Route the new OEM heated DEF line, ensuring it is not pinched or rubbing against sharp chassis components.

8. 8

   Reconnect the fluid fittings and the electrical connector.

9. 9

   Reconnect the battery, turn the key on, and use a diagnostic scan tool to clear the ECU fault codes. Run the machine to verify the code does not return.