Massey Ferguson 521003.03 – Engine Control Unit (ECU) / EEM4: DEF Module Heater Resistance Is Low (ECU) / DEF heater main relay control circuit short circuit to HIGH SOURCE

The engine control module has detected that the control circuit for the Diesel Exhaust Fluid (DEF) heater main relay is shorted to a high voltage source (such as battery voltage), or the resistance in the DEF module heater circuit is abnormally low, indicating a potential internal short.

• 4710
• 7724S
• 5712SL
• Interim Tier 4 (2012-2015)
• 7722
• 7722S
• 7615
• 5613
• 5713SL
• 7720S
• 7716
• 7718
• T4
• 7715
• 5710SL
• 7726
• 7616
• 7618
• 5608
• 4708
• 7619
• 7626
• 7726S
• 7624
• 4707
• 4709
• 5711SL
• 7614
• 7718S
• 5610
• 7620
• 7719
• 7714
• 7716S
• T4 (2015-2021)
• 5612
• 7622
• 7720
• Tier 4 (2015-2021)
• 5611
• 7715S
• 5609
• 7724
• 4700 Series
• MF 4707
• MF 4708
• MF 4709
• MF 4710
• T4 models (2015-2021)
• 5400 series

1. 1

   Perform a relay swap test: Locate the DEF heater main relay in the main electrical supply/fuse panel and swap it with a known-good relay of the same type.

2. 2

   Inspect control circuit wiring between the DEF heater main relay and the Engine ECU for a short to battery voltage (High Source) or chafed wires.

3. 3

   Check the resistance of the DEF module heater at the connector to ensure it is not internally shorted (which causes the low resistance condition).

4. 4

   If the relay and wiring are intact, verify operation with a test EEM4 controller to rule out an internal ECU fault.

1. 1

   Disconnect Power

   Disconnect the tractor's battery ground cable to ensure safety while working on the electrical system.

2. 2

   Access the Relay Panel

   Open the main electrical supply panel and identify the DEF heater main relay.

3. 3

   Inspect and Replace Relay

   Remove the relay and inspect the socket terminals for corrosion, melting, or damage. If the relay failed the swap test, replace it with a new OEM relay.

4. 4

   Repair Wiring

   If the relay is good but a short to high source was found during diagnostics, trace the wiring harness back to the ECU. Repair or replace the damaged section of the harness.

5. 5

   Replace DEF Heater

   If the DEF module heater itself has failed (low resistance), drain the DEF tank, disconnect the DEF lines and electrical connectors, remove the DEF supply module/heater assembly, and install a replacement unit.

6. 6

   Reassemble and Clear Codes

   Reconnect the battery. Use a diagnostic scan tool (such as the WinEEM3s/EEM4 Service Tool) to clear the DTCs. Run the engine and verify the fault does not return.