Massey Ferguson 4355.04 – Engine Control Unit (ECU): DEF Return Pipe Heater Relay Issue (ECU)

DEF backflow/return line heater relay high side (heater, main relay) has an open circuit or is shorted to ground (circuit failed low). This prevents the DEF return line from heating up, which can lead to frozen DEF fluid in cold conditions and subsequent emissions derates.

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

1. 1

   Ensure the engine is stopped and cooled down before starting work

2. 2

   Inspect the DEF heater wiring harness and connectors for physical damage, chafing, or melting

3. 3

   Check connector pins for the DEF return pipe heater and relay block for loose pins, pushed-out terminals, or debris/water

4. 4

   Inspect machine earths (grounds) for any damaged or loose connections

5. 5

   Check the DEF heater fuse (typically 25A in the emissions fuse block); a blown fuse strongly indicates a short to ground

1. 1

   System Reset

   Turn the ignition off for at least 60 seconds to allow the EEM4 system to power down and reset.

2. 2

   Locate the Relay

   Access the machine's electrical center and locate the DEF Heater Relay (often designated as K22 or similar, depending on the specific tractor model).

3. 3

   Test the Relay

   Remove the relay and test it with a multimeter. Check for an open circuit on the coil side or a short to ground on the high side. If the relay fails testing, replace it.

4. 4

   Test the Circuit

   If the relay is good, use a multimeter to measure the resistance of the wiring from the relay socket to the DEF return pipe heater. Check for continuity to ground (which would confirm the shorted to ground / failed low condition).

5. 5

   Repair Wiring

   If a short is found in the harness, trace the wire along the chassis to the DEF tank/pump area and repair the damaged section of the harness.

6. 6

   Clear Codes

   Reinstall the relay (or install a new one). Turn the key on and use a diagnostic service tool (such as the AGCO WinEEM3s/EEM4 Service Tool) to clear the fault code.

7. 7

   Verify Repair

   Cycle the key and verify the code does not return to active status.