Case IH 19070 (DTC 291A) - ECU – Engine Control Unit (ECU): Upstream SCR catalyst temperature too low

The Exhaust Gas Temperature (EGT) sensor located just before the Selective Catalytic Reduction (SCR) muffler is reporting a temperature value that is lower than the expected physical range or plausibility limit. When this sensor reads too low, the system will halt DEF dosing, trigger an SCR system failure warning, and eventually force the machine into a severe engine derate or 'limp home' mode.

• MAXXUM T4E
• 240 Series Axial-Flow (7240, 8240, 9240)
• Magnum Series (225, 235-340)
• Steiger Series (450, 620)
• Farmall CVT
• SV340
• TR340

1. 1

   Perform a Cold Soak Temperature Plausibility Test: Let the machine sit until the engine is completely cold. Check the readings of the Ambient Air Temperature sensor, the DOC temperature sensor, the Upstream SCR temperature sensor, and the Downstream SCR temperature sensor. They should all read the exact same ambient temperature. If the upstream sensor is significantly skewed, the sensor is faulty.

2. 2

   Inspect wiring and connectors: Inspect the wiring harness and the 2-pin connector going to the upstream SCR temperature sensor (Pin 1 is a 5V supply, Pin 2 is a common ground to the ECU). Look for chafing, melted wires, or corrosion that could cause high resistance.

3. 3

   Verify software versions: Check if the Engine ECU and the DNox unit have the latest software installed to address overly sensitive SCR temperature plausibility logic that can trigger false codes.

4. 4

   Check engine operating temperature: Ensure the engine itself is reaching proper operating temperature. A stuck-open engine coolant thermostat can prevent exhaust gases from getting hot enough, triggering this code.

1. 1

   Power Down

   Turn off the machine, remove the key, and disconnect the battery to prevent electrical spikes.

2. 2

   Locate the Sensor

   Locate the Upstream SCR Temperature Sensor. It is threaded into the exhaust pipe directly before the main SCR catalyst/muffler chamber.

3. 3

   Disconnect Wiring

   Unplug the electrical connector linking the sensor to the engine harness.

4. 4

   Remove the Sensor

   Using a specialized sensor socket or a line wrench, carefully unscrew the sensor from the exhaust pipe. (Note: Exhaust sensors are prone to seizing due to extreme heat cycles. Apply penetrating oil and allow it to soak if the sensor is stuck to avoid stripping the exhaust bung threads).

5. 5

   Install the New Sensor

   Apply a small amount of high-temp anti-seize compound to the threads of the new sensor (if it does not come pre-coated). Thread it in by hand to prevent cross-threading, then tighten to the OEM torque specification.

6. 6

   Reconnect

   Plug the sensor's electrical connector back into the harness and reconnect the battery.

7. 7

   Reset the System

   Connect the Case IH Electronic Service Tool (EST) laptop to clear the fault codes and reset the SCR system logic, as simply replacing the sensor will not clear the derate.