Case IH 19766 – DCU (Dosing Control Unit): Compressed air regulation valve (19766 / P20A7) and Catalyst temperature plausibility (19775 / P0426)

The dossier addresses two distinct fault codes: 19766 (P20A7) for a compressed air regulation valve error in the air control group, and 19775 (P0426) for a catalyst temperature plausibility error where the ratio between downstream and upstream temperature sensors is below the acceptable threshold.

• Tier 4a
• Magnum 235 – 340 (Series 4)
• Steiger 350 – 600
• Quadtrac 450 – 600
• Puma series
• Optum series

1. 1

   For Catalyst Temperature Plausibility (P0426 cause): Visually inspect that both the upstream (before catalyst) and downstream (after catalyst) exhaust gas temperature sensors are installed correctly and securely.

2. 2

   For Catalyst Temperature Plausibility (P0426 cause): Inspect the muffler and exhaust gas piping for physical damage, cracks, or exhaust leaks that could introduce ambient air or allow heat to escape.

3. 3

   For Catalyst Temperature Plausibility (P0426 cause): Use the Case IH Electronic Service Tool (EST) to monitor live data of the temperature sensors on a completely cold tractor (e.g., sat overnight). Both sensors should read very close to ambient air temperature.

4. 4

   For Compressed Air Regulation Valve (19766 / P20A7 cause): Check the wiring harness and connector at the compressed air regulation valve for corrosion, backed-out pins, or broken wires.

5. 5

   For Compressed Air Regulation Valve (19766 / P20A7 cause): Verify that the DEF dosing system is receiving adequate air pressure from the tractor's main air supply.

1. 1

   Identify faulty sensor

   For Catalyst Temperature Plausibility (P0426 cause): Identify the faulty sensor (upstream or downstream) based on the EST live data comparison.

2. 2

   Disconnect battery

   For Catalyst Temperature Plausibility (P0426 cause): Turn off the machine and disconnect the negative battery terminal.

3. 3

   Locate sensor

   For Catalyst Temperature Plausibility (P0426 cause): Locate the faulty Exhaust Gas Temperature (EGT) sensor on the SCR/Catalyst housing.

4. 4

   Disconnect connector

   For Catalyst Temperature Plausibility (P0426 cause): Disconnect the electrical connector for the sensor.

5. 5

   Remove sensor

   For Catalyst Temperature Plausibility (P0426 cause): Using a suitable wrench or O2/sensor socket, carefully remove the sensor from the exhaust pipe (penetrating oil may be required if the threads are seized due to heat cycling).

6. 6

   Apply anti-seize

   For Catalyst Temperature Plausibility (P0426 cause): Apply a small amount of high-temp anti-seize compound to the threads of the new sensor (ensure absolutely no compound gets on the sensor probe itself).

7. 7

   Install sensor

   For Catalyst Temperature Plausibility (P0426 cause): Install the new sensor and torque to OEM specifications.

8. 8

   Reconnect components

   For Catalyst Temperature Plausibility (P0426 cause): Reconnect the electrical connector and battery.

9. 9

   Clear codes

   For Catalyst Temperature Plausibility (P0426 cause): Clear the fault codes using the EST. Note: The code may remain in a pending state until the engine is brought up to full operating temperature and run under load, allowing the DCU to verify that the SCR system is operating properly.

10. 10

    Relieve air pressure

    For Compressed Air Regulation Valve (19766 / P20A7 cause): If the wiring is intact but the valve fails to actuate or hold pressure during an EST component test, relieve the system air pressure.

11. 11

    Disconnect valve

    For Compressed Air Regulation Valve (19766 / P20A7 cause): Disconnect the air lines and electrical connector from the compressed air regulation valve.

12. 12

    Replace valve

    For Compressed Air Regulation Valve (19766 / P20A7 cause): Remove the mounting hardware and replace the valve.

13. 13

    Reconnect and test

    For Compressed Air Regulation Valve (19766 / P20A7 cause): Reconnect the lines, power up the system, check for air leaks, and clear the DCU codes.