Case IH 3771 (F-4765-31) – Engine Control Module (ECM) / Engine ECU: Diesel Oxidation Catalyst Up and down stream temperature sensors are swapped

The engine control system has detected that the temperature readings for the inlet (upstream) and outlet (downstream) of the Diesel Oxidation Catalyst (DOC) are inverted, almost always because the electrical connectors for the two sensors were accidentally plugged into the wrong sensors during a recent service or repair.

• VESTRUM CVX
• Farmall 40
• Farmall 45
• Farmall 50
• Farmall 115U
• Maxxum 140
• Magnum 290
• Steiger series
• SV250

1. 1

   Inspect the wiring harness leading to the Diesel Oxidation Catalyst (DOC) on the exhaust system and trace the wires to both the upstream and downstream temperature sensors.

2. 2

   Verify if the machine recently had exhaust, DPF, or DOC maintenance performed, as this code is typically triggered by human error during reassembly.

3. 3

   Connect the Electronic Service Tool (EST) and monitor live data for DOC upstream and downstream temperatures during a cold start. If the downstream temperature rises significantly faster than the upstream temperature before the catalyst has warmed up, the sensors are physically plugged into the wrong harness connectors.

1. 1

   Locate the Sensors

   Identify the Diesel Oxidation Catalyst (DOC) in the exhaust aftertreatment system. Locate the two temperature sensors (one before the DOC and one after the DOC).

2. 2

   Disconnect the Harness

   Carefully unplug the electrical connectors from both the upstream and downstream temperature sensors.

3. 3

   Swap the Connections

   Reroute and plug the upstream wiring harness connector into the upstream sensor, and the downstream wiring harness connector into the downstream sensor.

4. 4

   Clear the Code

   Connect the Electronic Service Tool (EST) and clear the active 3771 fault code.

5. 5

   Test Run

   Start the engine and monitor the live temperature data to ensure the upstream sensor heats up first and the readings are now plausible. Ensure the fault code does not return.