Massey Ferguson 521001.05 – Engine ECU: DEF Module Heater Resistance Is High (ECU)

The system has detected that the electrical resistance in the Diesel Exhaust Fluid (DEF) module heater circuit is higher than the acceptable threshold. This typically indicates an open circuit, which is usually caused by a burnt-out heating element inside the DEF supply module (pump), a broken wire, or a severely corroded electrical connector.

• 4710
• 7724S
• 5712SL
• 7722S
• 7722
• 5713SL
• 7720S
• 7716
• 7718
• T4
• 7715
• 5710SL
• 7726
• 4708
• 7726S
• 4707
• 4709
• 5711SL
• 7718S
• 7719
• 7714
• 7716S
• T4 (2015-2021)
• 7720
• Tier 4 (2015-2021)
• 7715S
• 7724
• 4700 Series
• 5600 Series
• 7600 Series
• 8600 Series

1. 1

   Inspect the main electrical supply panel and test the DEF Module Heater Relay (R5) and associated fuses to ensure power is being sent to the module.

2. 2

   Visually inspect the wiring harness from the ECU to the DEF supply module for chafed wires, rodent damage, or loose connections.

3. 3

   Disconnect the plug at the DEF pump and check for corrosion or pushed-out pins.

4. 4

   Using a digital multimeter, measure the resistance (Ohms) directly across the heater circuit pins on the DEF supply module. An 'OL' (open line) or extremely high resistance indicates a failed internal heating element.

1. 1

   Prep the Machine

   Turn off the engine, remove the key, and disconnect the battery ground cable.

2. 2

   Locate the DEF Pump

   Locate the DEF supply module, which is typically mounted near the DEF tank.

3. 3

   Disconnect Electricals

   Unplug the main electrical harness connector from the DEF module.

4. 4

   Disconnect Fluid Lines

   Carefully disconnect the DEF suction, pressure, and return lines. Note: DEF is corrosive to metal and wiring. Catch any spilled fluid with a rag and wash the area immediately with warm water.

5. 5

   Remove the Module

   Unbolt the mounting hardware securing the DEF pump to its bracket and remove the unit.

6. 6

   Install the New Module

   Mount the replacement DEF supply module and tighten the bolts to the manufacturer's torque specifications.

7. 7

   Reconnect

   Reattach the DEF fluid lines (ensuring they click/lock securely into place) and plug in the electrical connector.

8. 8

   Clear Codes and Prime

   Reconnect the battery. Use a diagnostic scan tool to clear the 521001.05 fault code. Turn the key to the ON position to allow the DEF pump to prime the system, and check for any fluid leaks before starting the engine.