What is XCMG XE750D Fault Code 3405-6 SPN2791 FMI6?

Fault Code 3405-6 (SPN 2791, FMI 6) indicates a calibration or configuration error in the diesel exhaust fluid (DEF) pump system on the XCMG XE750D excavator. This diagnostic trouble code (DTC) specifically points to an issue where the Engine Control Module (ECM) has detected that the DEF pump current or voltage is out of the expected calibration range or the component has incorrect configuration data.

The SPN 2791 refers to the DEF pump control circuit, while FMI 6 indicates a current above normal or grounded circuit condition. On the XE750D's aftertreatment system, the DEF pump is critical for injecting precise amounts of diesel exhaust fluid into the Selective Catalytic Reduction (SCR) system to reduce NOx emissions. When this calibration fault occurs, the excavator's emissions control system cannot function properly, potentially leading to engine derate or shutdown to protect the environment and comply with emissions regulations.

For used XCMG excavators, this fault is particularly significant because improper repairs, aftermarket DEF pump replacements, or ECM software mismatches from previous service work can trigger calibration errors that weren't present when the machine was new.

Common Symptoms

• Amber or red engine malfunction indicator lamp (MIL) illuminated on the dashboard
• Engine power derate (reduced horsepower and hydraulic performance) after a grace period
• DEF system warning messages displayed on the operator monitor
• Increased exhaust smoke or unusual exhaust odor due to incomplete SCR function
• Potential engine shutdown if the fault persists beyond the manufacturer's grace period for emissions-related codes

Potential Causes

• Incorrect DEF pump calibration following ECM replacement or software update
• Aftermarket or incompatible DEF pump installed during previous repairs on used equipment
• Corrupted ECM configuration data due to low battery voltage during programming
• Wiring harness damage at the DEF pump connector, especially at routing points near the chassis frame where vibration causes chafing
• Corrosion on DEF pump connector pins, common in excavators exposed to harsh environments or DEF spillage
• Failed DEF pump internal electronics requiring recalibration or replacement
• ECM software version mismatch with installed aftertreatment components

How to Troubleshoot and Fix Code 3405-6 SPN2791 FMI6

Step 1: Connect Diagnostic Software Use XCMG-approved diagnostic software or compatible heavy-duty scan tools to read the active and inactive fault codes. Verify the exact SPN and FMI, and check for additional related codes in the aftertreatment system. Document the freeze frame data showing operating conditions when the fault occurred.

Step 2: Inspect DEF Pump and Wiring Harness Physically examine the DEF pump connector for corrosion, bent pins, or DEF crystallization (white residue). On used excavators, check the wiring harness routing from the pump to the ECM for signs of rubbing against the frame, exhaust components, or hydraulic lines. Use a digital multimeter to test for continuity and proper resistance values (typically 3-8 ohms for the pump circuit, but verify with XCMG specifications).

Step 3: Verify DEF Pump Calibration and Configuration Access the ECM calibration files through diagnostic software and confirm the DEF pump part number matches the configuration stored in the ECM. If a replacement pump was installed on this used machine, recalibrate the system using manufacturer procedures. Clear fault codes and perform a DEF pump prime cycle to verify proper operation.

Step 4: Test DEF Pump Current Draw With the ignition on and pump activated through diagnostic commands, measure the current draw at the pump connector. Typical values range from 8-12 amps during operation. Values significantly higher suggest internal pump failure or grounded circuits, while lower values indicate open circuits or poor connections.

Step 5: Perform ECM Software Update if Necessary If calibration mismatches persist, the ECM may require a software reflash with the latest XCMG updates. This is especially common on used excavators that have had component replacements without proper system reconfiguration. Ensure stable battery voltage (above 12.5V) during programming to prevent corruption.

Critical for Used Equipment: Before replacing expensive components like the DEF pump or ECM, thoroughly inspect all connectors and harnesses for wear patterns typical of high-hour machines. Many calibration faults on used excavators stem from poor connections or previous improper repairs rather than actual component failure.

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Disclaimer: This guide provides general troubleshooting information for fault code 3405-6 on XCMG XE750D excavators. Always consult the official XCMG service manual and consider professional diagnostic assistance for complex emissions system repairs. Improper repairs to aftertreatment systems may result in regulatory non-compliance and further equipment damage.