What is XCMG XE750D Fault Code 3391-6 SPN5425 FMI6?

Fault Code 3391-6 (SPN 5425, FMI 6) indicates a communication or calibration error with the Diesel Exhaust Fluid (DEF) pump or injector system, specifically pointing to a voltage or data signal that is outside the normal operating range or experiencing intermittent connectivity issues.

This code is part of the Selective Catalytic Reduction (SCR) aftertreatment system used in XCMG XE750D excavators to meet emissions standards. SPN 5425 relates to the DEF system's dosing control, while FMI 6 signals that the Engine Control Module (ECM) is detecting data values that are abnormally high or experiencing communication dropout. On this model, SCR system faults can trigger engine derating or prevent the machine from starting after extended shutdown periods, making prompt diagnosis critical for maintaining productivity.

Common Symptoms

• Amber or red warning light on the instrument panel indicating SCR/emissions system fault
• Engine power derate (reduced horsepower) after a certain period, potentially dropping to 50% power
• Intermittent loss of communication between the ECM and DEF dosing module
• DEF consumption abnormalities – either no DEF usage or excessive consumption
• Engine may enter limp mode or refuse to restart after cool-down on used machines with accumulated fault history

Potential Causes

The most common technical causes for this fault code on used XCMG XE750D excavators include:

• Corroded or damaged wiring harness connections at the DEF pump or injector module (common in machines operating in wet or corrosive environments)
• Failed DEF pump motor or control module due to crystallized DEF contamination from poor-quality fluid or extended storage
• Intermittent voltage supply issues from the machine's main electrical system or battery connections
• ECM software calibration mismatch after component replacement without proper reprogramming
• DEF injector nozzle blockage causing back-pressure that affects sensor readings
• Damaged CAN bus communication lines between the SCR system and ECM, particularly at harness rub points near the engine firewall

How to Troubleshoot and Fix Code 3391-6 SPN5425 FMI6

Step 1: Visual Inspection and Connection Check Begin by inspecting the DEF pump wiring harness for visible damage, corrosion, or loose connections. On used excavators, pay special attention to harness routing near moving components or heat sources. Clean all electrical connectors with electrical contact cleaner and apply dielectric grease. Check the DEF tank level and fluid quality – contaminated or crystallized DEF is common in older machines.

Step 2: Electrical Testing Using a digital multimeter, test the voltage supply to the DEF pump module. You should see approximately 12-24V (depending on system design) at the pump connector with ignition on. Check ground continuity – poor grounds are frequent culprits in used equipment. Use XCMG's diagnostic software or a compatible scan tool to monitor live data from the DEF system, looking for intermittent signal dropout.

Step 3: Component Testing and Replacement Test the DEF pump motor resistance according to XCMG specifications (typically 3-8 ohms). If readings are out of range or the pump fails to activate during commanded tests, replacement is necessary. Before installing new components on used machines, thoroughly flush the DEF system to remove crystallized deposits. After any component replacement, ensure the ECM is reprogrammed with the latest calibration files to prevent recurrence.

Step 4: CAN Bus Communication Verification If previous steps don't resolve the issue, test the CAN bus communication lines between the SCR module and ECM using an oscilloscope or advanced diagnostic tool. Look for proper signal voltage (typically 2.5V nominal) and check for short circuits or open circuits in the twisted-pair wiring.

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Disclaimer: This guide provides general troubleshooting information. Always consult the XCMG XE750D service manual and consider engaging a certified heavy equipment technician for complex electrical diagnostics. Improper repairs to emissions systems may violate environmental regulations.