Case IH F-5841-12 - ECU – Engine Control Unit (ECU): DEF/AdBlue quality sensor internal failure (Internal fault)

The DEF quality sensor's internal circuitry, logic, or ultrasonic measuring component has suffered a hard hardware failure, rather than simply detecting poor-quality fluid.

• MAGNUM AFS Connect
• 240 Series Axial-Flow Combines
• Magnum
• Steiger
• Farmall series
• Farmall 110A
• SV340B

1. 1

   Check for accompanying CAN bus communication errors or voltage codes that might falsely flag an internal failure

2. 2

   Disconnect the DEF header unit and inspect the pins for corrosion or moisture intrusion

3. 3

   Verify the wiring harness has no chafing or breaks

4. 4

   Ensure the sensor is receiving the correct reference voltage and ground from the ECU/ACM

5. 5

   Use a DEF refractometer to manually verify the DEF concentration is between 32.5% and 33% to rule out physical damage from degraded fluid

1. 1

   Preparation

   Turn off the machine, disconnect the battery, and thoroughly clean the top of the DEF tank around the header unit to prevent dirt and debris from falling into the tank.

2. 2

   Disconnect Electricals

   Unplug the electrical connector from the DEF header unit.

3. 3

   Disconnect Lines

   Carefully disconnect the DEF suction and return lines. If the machine is equipped with coolant-heated DEF lines, clamp and disconnect the engine coolant lines attached to the header. Catch any spilled fluid with clean rags.

4. 4

   Remove the Header

   Unscrew the locking ring or remove the mounting bolts securing the DEF header to the tank. Carefully pull the entire DEF header assembly straight up and out of the tank.

5. 5

   Install the New Part

   Insert the new DEF header assembly into the tank, ensuring the new mounting gasket/O-ring is properly seated. Tighten the locking ring or bolts to the OEM torque specification.

6. 6

   Reconnect

   Reattach the DEF lines, coolant lines, and the electrical connector.

7. 7

   Refill

   Refill the tank with fresh, high-quality DEF. Reconnect the battery.

8. 8

   Clear Codes and Test

   Use a diagnostic scan tool (such as the CNH Electronic Service Tool) to clear the F-5841-12 fault code. Run an SCR system test or command a parked regeneration to ensure the ECU recognizes the new sensor and the system operates without derating.