Case IH 3994 – Engine ECU (ISM-IDI ENGINE controller): Diesel Oxidation Catalyst (DOC) IN Temperature is over 800 °C (1472 °F)

The exhaust gas temperature entering the Diesel Oxidation Catalyst (DOC) has exceeded the safe operating threshold of 800 °C (1472 °F). This indicates either the exhaust system is genuinely overheating, or the Catalyst Inlet Temperature Sensor has failed/shorted and is sending a false high reading to the computer.

• skid steer
• Steiger
• Quadtrac
• Magnum
• Maxxum
• Puma
• Optum
• Farmall
• Axial-Flow 5130
• Axial-Flow 7240
• Axial-Flow 9250
• Skid Steer Loaders (SSL)
• Compact Track Loaders (CTL)

1. 1

   Live Data Verification: Connect the Case IH Electronic Service Tool (EST) and monitor the DOC Intake Temperature live data. If the sensor reads 800+ °C when the engine is completely cold, the sensor or its wiring is definitively shorted/faulty.

2. 2

   Wiring Harness Inspection: Inspect the wiring harness leading to the DOC intake temperature sensor. Look for melted wires, chafing against the exhaust pipe, or a short to ground/voltage.

3. 3

   Connector Check: Disconnect the sensor and check the pins for corrosion, moisture, or heat damage.

4. 4

   Mechanical Exhaust Check: If the sensor reads normally when cold but spikes rapidly when running, investigate the engine for conditions that cause excessive exhaust heat, such as a stuck-open fuel injector dumping raw fuel into the exhaust, or a severely restricted DPF/exhaust system.

1. 1

   Cool Down

   Turn off the engine and allow the exhaust system to cool completely. Exhaust components operate at extreme temperatures and pose a severe burn risk.

2. 2

   Locate the Sensor

   Locate the DOC Intake Temperature Sensor (also called the Catalyst Inlet Temp Sensor). It is threaded into the exhaust pipe immediately upstream of the Diesel Oxidation Catalyst canister.

3. 3

   Disconnect

   Unplug the electrical connector from the sensor pigtail.

4. 4

   Removal

   Using a properly sized wrench or sensor socket, carefully unscrew the sensor from the exhaust pipe. Note: Exhaust sensors frequently seize due to heat cycling. You may need to apply penetrating oil and let it sit before attempting removal to avoid stripping the exhaust bung threads.

5. 5

   Preparation

   Apply a small amount of high-temperature anti-seize compound to the threads of the new sensor (if it does not come pre-applied from the factory). Do not get anti-seize on the sensor probe itself.

6. 6

   Installation

   Thread the new sensor into the exhaust bung by hand to prevent cross-threading, then tighten it securely with a wrench.

7. 7

   Reconnect

   Plug the electrical connector back into the engine harness and secure any loose wire routing away from direct heat sources.

8. 8

   Clear Codes

   Use your diagnostic tool to clear fault code 3994 from the Engine ECU and run the machine to verify the temperature reads within normal operating parameters.