Case IH 3771 – ECM: DOC Temperature Sensors Swapped

This code indicates a plausibility error detected by the engine computer. The downstream Diesel Oxidation Catalyst (DOC) temperature is rising faster or reading significantly hotter than the upstream temperature, which defies thermodynamics. This almost exclusively happens after a recent repair, exhaust service, or engine swap where a technician accidentally plugged the upstream temperature sensor into the downstream wiring harness connector, and vice versa.

• skid steer
• Magnum
• Puma
• Maxxum
• Vestrum
• Skid Steer Loaders (SSL)
• Compact Track Loaders (CTL)
• Wheel Loaders
• Excavators

1. 1

   Verify if the machine recently had exhaust work, a DPF/DOC cleaning, sensor replacement, or an engine swap.

2. 2

   Connect a diagnostic scan tool and monitor the live data parameters for DOC Upstream Temperature and DOC Downstream Temperature.

3. 3

   Start the engine from a cold state. If the downstream temperature rises first, the connectors are definitively crossed.

4. 4

   Trace the wiring from the two temperature sensors on the DOC housing to the main engine harness, checking for physical signs of crossed wires, stretched harnesses, or forced connectors.

1. 1

   Cool exhaust system

   Turn off the engine, remove the key, and allow the exhaust system (DOC/DPF assembly) to cool completely to avoid severe burns.

2. 2

   Locate DOC and sensors

   Locate the Diesel Oxidation Catalyst (DOC) in the exhaust aftertreatment assembly and identify the upstream and downstream Exhaust Gas Temperature (EGT) sensors.

3. 3

   Disconnect sensors

   Disconnect the wiring harness connectors for both the upstream and downstream sensors.

4. 4

   Swap connections

   Swap the harness connections: plug the upstream sensor into the upstream harness lead, and the downstream sensor into the downstream harness lead. Ensure they are routed correctly without tension.

5. 5

   Clear fault code

   Reconnect the batteries or turn the key on, and use the CNH EST diagnostic tool to clear the 3771 fault code.

6. 6

   Verify repair

   Start the engine and monitor the DOC temperature live data to verify the upstream sensor is now reading hotter than the downstream sensor during the warm-up phase.