Case IH 19865 – Engine Control Unit (ECU) or Dosing Control Unit (DCU) / Heater Control Unit (HCU): Short Circuit to Battery of DEF/AdBlue Backflow Line Heater Actuator powerstage

The control module has detected an abnormally high voltage (a short to a 12V battery source) on the electrical circuit that powers the heater for the DEF (Diesel Exhaust Fluid) backflow line. The backflow line is responsible for returning unused DEF from the dosing module/pump back to the DEF tank, and its integrated heater prevents the fluid from freezing in cold temperatures.

• Tier 4b
• Steiger
• Magnum
• Puma
• 580N T4A
• N-series Tractor Loader Backhoes

1. 1

   Visual Inspection: Park the machine, turn the ignition OFF, and locate the DEF backflow line. Inspect the wiring harness leading to the backflow line heater for chafing, melting, or pinched wires that could cause the heater control wire to touch a 12V power supply wire.

2. 2

   Connector Check: Disconnect the DEF backflow line heater harness connector. Inspect the pins for moisture, corrosion, or damaged terminals.

3. 3

   Voltage Test: With the connector unplugged, turn the ignition ON. Use a multimeter to measure the voltage at the harness side of the connector. If it reads a constant 12 Volts when the heater is not being commanded on, the wiring harness is shorted to battery voltage.

4. 4

   Relay Check: Locate the DEF line heater relay in the fuse/relay center. A relay that is internally stuck closed will continuously send battery voltage to the heater circuit. Swap the relay with a known good one of the same type to see if the short circuit clears.

5. 5

   Component Resistance Test: Test the resistance of the DEF backflow line heater element itself. If the internal heating element is shorted, the entire line must be replaced.

1. 1

   Secure the Machine

   Park on a level surface, turn the ignition OFF, set the parking brake, and disconnect the negative battery cable to prevent electrical spikes.

2. 2

   Address Wiring Issues (If Found)

   If the diagnostic tests revealed a short in the wiring harness, open the wire loom and locate the chafed section. Repair the damaged wires using OEM-approved heat-shrink connectors, or replace the affected harness section.

3. 3

   Replace the Relay (If Faulty)

   If the DEF heater relay was found to be stuck closed, remove the old relay from the fuse panel and install a new OEM relay.

4. 4

   Replace the DEF Backflow Line (If Internally Shorted)

   Clean the area around the DEF pump and tank connections to prevent dirt from entering the SCR system. Place a catch pan under the connections to catch any draining DEF. Disconnect the electrical connector for the backflow line heater. Disconnect the fluid fittings at the DEF pump and the DEF tank. Cap the open ports immediately to prevent DEF crystallization. Route the new heated DEF backflow line in the exact same orientation as the old one, ensuring it does not rub against sharp frame edges. Connect the fluid fittings and plug in the electrical connector.

5. 5

   Clear Codes and Test

   Reconnect the battery. Turn the key ON and use a diagnostic scan tool (such as the CNH Electronic Service Tool) to clear the inactive fault codes. Start the engine and run a stationary SCR system test to verify the heater operates correctly and the code does not return.