Massey Ferguson 521001.04 – Engine Control Unit (ECU): DEF Module Heater Resistance Is Low (ECU) / Low Voltage Issue With DEF Heater Relay Latching Supply Voltage (ECU)

The system has detected that the electrical resistance in the Diesel Exhaust Fluid (DEF) module heater circuit is abnormally low, or there is a low voltage condition on the latching supply voltage for the DEF heater relay. A low resistance reading typically indicates a short circuit in the wiring, a shorted heater element within the DEF module, or a failing relay.

• 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

1. 1

   Inspect the DEF supply module heater relay wiring and connectors for oxidation, corrosion, or pushed-out pins.

2. 2

   Swap the DEF supply module heater relay with a known-good relay of the same type to test for a defective relay.

3. 3

   Use a multimeter to measure the resistance of the DEF module heater element to confirm if the internal resistance is low (shorted) or if the issue is in the wiring harness.

4. 4

   Check the ECU operation and ensure the wiring harness between the ECU and the DEF heater relay does not have a short to ground.

1. 1

   Clear the Log

   Save the current error log in the diagnostic tool, then erase the codes.

2. 2

   Power Down

   Turn off the tractor and disconnect the battery to ensure safety while working on the electrical system.

3. 3

   Relay Replacement

   If the diagnostic checks proved the relay was faulty, replace the DEF Module Heater Relay.

4. 4

   Harness Repair

   If oxidation or a short was found in the wiring harness or connector pins during the diagnostic phase, clean the contacts with electrical contact cleaner or repair the damaged wire sections.

5. 5

   Module Replacement

   If the relay and wiring are good, but the DEF module heater element itself tests out of specification (low resistance), the DEF Supply Module / Heater must be replaced.

6. 6

   Reboot and Test

   Reconnect the battery. Restart the ECU after erasing the error log. Operate the tractor through a standard driving cycle to ensure the fault code does not return.