Case IH 3840 – Engine Control Unit (ECU): Diesel Particulate filter upstream temperature sensor: Diagnostic fault check for SRC high

"SRC high" (Source High) indicates that the voltage signal returning from the temperature sensor to the computer is exceeding the maximum expected threshold. Because these are typically thermistor-style sensors, a high voltage reading almost always points to an open circuit (broken wire, disconnected plug, or burned out sensor).

• Farmall
• skid steer
• Magnum
• Puma
• Maxxum
• Vestrum
• Quadtrac
• Quantum
• AFX / Axial-Flow
• 8240
• 8250
• 6150
• SV250
• SR175

1. 1

   Visual Inspection: Inspect the wiring harness leading to the DPF upstream temperature sensor. Look for melted wires (due to exhaust heat), chafing against the chassis, or rodent damage.

2. 2

   Connector Check: Unplug the sensor and inspect the connector pins. Look for pushed-back pins, corrosion, or water intrusion.

3. 3

   Sensor Resistance Test (Sensor failure cause): Using a multimeter, measure the resistance across the pins of the sensor itself. If the meter reads OL (Open Loop) or infinite resistance, the sensor has failed internally and must be replaced.

4. 4

   Harness Voltage Test (Wiring/ECU cause): With the sensor unplugged and the machine's key in the ON position (engine off), measure the voltage at the harness-side connector. You should typically see a 5V reference signal. If there is no voltage, or if it reads 12V (short to battery), the wiring harness or ECU is the culprit.

1. 1

   Preparation

   Turn off the machine, remove the key, and allow the exhaust system to cool completely to avoid severe burns. Disconnect the negative battery cable.

2. 2

   Locate the Sensor

   Locate the DPF upstream temperature sensor. It is threaded into the exhaust pipe directly before the Diesel Particulate Filter (DPF) housing.

3. 3

   Removal

   Disconnect the electrical pigtail. Spray penetrating oil on the sensor threads and let it soak, as exhaust sensors frequently seize due to extreme heat cycles. Use a flare nut wrench or a specialized exhaust temperature sensor socket to carefully unscrew the sensor.

4. 4

   Installation

   Apply a small amount of high-temperature anti-seize compound to the threads of the new sensor (ensure no anti-seize gets on the sensor probe itself). Thread it in by hand to prevent cross-threading, then tighten to the manufacturer's torque specification.

5. 5

   Reconnection

   Plug the electrical connector back into the harness until it clicks. Secure the wiring with high-temp zip ties, ensuring it is routed safely away from the hot exhaust pipe.

6. 6

   Clear Codes & Verify

   Reconnect the battery. Use a compatible diagnostic tool (such as CNH EST or Jaltest) to clear the inactive 3840 fault code. Start the engine and monitor the exhaust temperature live data to ensure the sensor is reading correctly. A stationary forced DPF regeneration may be required to fully verify the system.