Case IH 19525 (DTC 26AA) - ECU – Engine ECU / EDC: DEF/AdBlue temperature sensor voltage is lower than expected

The signal voltage returning from the Diesel Exhaust Fluid (DEF) temperature sensor has dropped below the minimum acceptable threshold, indicating a short to ground in the sensor circuit, a severely damaged wiring harness, or a complete internal failure of the sensor's thermistor.

• MAXXUM T4E
• 240 Series Axial-Flow Combines (7240, 8240, 9240)
• Magnum Series Tractors (235-340)
• Steiger/Quadtrac
• Farmall Series
• Austoft 9900 sugarcane harvesters

1. 1

   Visual Harness Inspection: Trace the wiring harness from the DEF tank sensor to the ECU. Look for chafed wires, cuts, or areas where the harness may be rubbing against the frame and shorting to ground.

2. 2

   Connector Check: Disconnect the plug at the DEF header unit. Check for water ingress, DEF fluid crystallization (which is highly corrosive to pins), or bent pins.

3. 3

   Voltage/Ground Test: With the key ON (engine off), use a multimeter to verify that the sensor is receiving the correct 5V reference voltage and has a solid ground from the ECU.

4. 4

   Component Isolation: If the wiring and voltages are good, the internal temperature sensor inside the DEF header has likely failed and is shorting out the signal.

1. 1

   Prep the Machine

   Turn off the ignition and disconnect the battery to prevent electrical spikes. Clean the area around the top of the DEF tank thoroughly to prevent dirt from falling into the tank.

2. 2

   Disconnect Electricals

   Unplug the main electrical harness connector from the DEF header unit.

3. 3

   Disconnect Fluid Lines

   Disconnect the DEF supply and return lines from the header. Note: Cap the lines immediately to prevent contamination, as the SCR system is highly sensitive to dirt.

4. 4

   Remove the Header

   Remove the mounting screws or the locking ring securing the DEF header to the tank. Carefully pull the long header assembly straight up and out of the tank.

5. 5

   Install the New Unit

   Insert the new DEF header unit into the tank. Always use a new mounting seal/O-ring to prevent leaks. Secure the locking ring or screws.

6. 6

   Reconnect

   Reattach the DEF fluid lines and plug in the electrical connector.

7. 7

   Clear Codes

   Reconnect the battery. Turn the key ON and use a diagnostic scan tool (like the Case IH EST) to clear code 19525 / 26AA. Run the engine and monitor the DEF temperature data stream to verify the repair.