Massey Ferguson 4355.03 – Engine Control Unit (ECU) / EEM4: DEF backflow line heater relay low side (line relay) open circuit

The engine controller detects a break or 'open circuit' on the low side (ground control circuit) of the relay responsible for turning on the heater in the DEF return/backflow pipe.

• 7616
• 5612
• 7622
• Interim Tier 4 (2012-2015)
• 7614
• 5611
• 7615
• 5610
• 5613
• 7620
• 5609
• 7618
• 5608
• 7619
• 7626
• 7624
• 4707
• 4708
• 4709
• 4710
• 5710SL
• 5711SL
• 5712SL
• 5713SL
• 7714
• 7715
• 7716
• 7718
• 7719
• 7720
• 7722
• 7724
• 7726
• 8600 Series
• 8700 Series

1. 1

   Check the main fuse box for any blown fuses related to the DEF/AdBlue heating system or emissions controls

2. 2

   Locate the DEF return line heater relay and swap it with a known good relay of the same amperage, or bench-test it for internal open circuits

3. 3

   Inspect the wiring harness between the DEF heater relay and the Engine ECU, specifically on the low (ground) side of the relay control circuit, for chafed wires, broken connections, or backed-out pins

4. 4

   Check the quick connectors and O-rings on the DEF return line for corrosion, moisture ingress, or physical damage

1. 1

   Power Down

   Turn off the tractor and disconnect the negative battery terminal to prevent electrical shorts during repair.

2. 2

   Relay Replacement

   If the diagnostic checks prove the relay is faulty (internal open circuit), remove the old relay from the fuse/relay panel and install a new OEM replacement.

3. 3

   Harness Repair

   If the relay is functional but the wiring is broken, trace the low-side control wire from the relay block back to the ECU. Repair any broken wires using weather-sealed crimp connectors or replace the damaged harness section.

4. 4

   Line Replacement

   If the heater element integrated into the DEF return line has failed and caused a short/open that damaged the relay, replace the entire DEF return line assembly.

5. 5

   Reassembly and Clearing

   Reconnect the battery. Use a diagnostic tool (such as the WinEEM4 Service Tool) to clear the 004355.03 / SPN 4355 code. Run the engine and verify the code does not return.