What is XCMG XE750D Fault Code 2606-5 SPN3661 FMI5?

Fault Code 2606-5 (SPN 3661 FMI 5) indicates a current below normal or open circuit condition in the Diesel Exhaust Fluid (DEF) heater circuit. This code is part of the J1939 diagnostic protocol used in modern diesel engines equipped with Selective Catalytic Reduction (SCR) systems.

On the XCMG XE750D excavator, this fault specifically affects the DEF tank heating system, which is critical for maintaining proper DEF fluid temperature in cold weather operations. The DEF heater prevents the urea solution from freezing (DEF freezes at 12°F/-11°C) and ensures optimal SCR system performance. When this circuit fails, the machine may enter derate mode or prevent engine startup in cold conditions, significantly impacting productivity on job sites.

Common Symptoms

• Amber or red warning light illuminated on the instrument cluster indicating SCR system malfunction
• Engine power derate (reduced horsepower) after a countdown timer expires, typically limiting machine to 40-60% power
• DEF quality or level warnings displayed on the monitor, even when fluid levels are adequate
• Hard starting or no-start condition in freezing temperatures due to frozen DEF
• Frequent DEF system fault messages appearing on the operator display

Potential Causes

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

• Failed DEF heater element due to age, corrosion, or electrical burnout from voltage spikes
• Open circuit in wiring harness between the Engine Control Module (ECM) and DEF heater, often caused by harness chafing near the tank mounting bracket
• Corroded or loose electrical connectors at the DEF tank heater, particularly on machines operated in harsh or marine environments
• Damaged wiring from rodent activity or physical damage during maintenance work around the DEF tank area
• Faulty ECM driver circuit for the heater control (less common but possible on higher-hour machines)
• Blown fuse or relay in the DEF heater power supply circuit

How to Troubleshoot and Fix Code 2606-5 SPN3661 FMI5

Step 1: Visual Inspection Begin by thoroughly inspecting the DEF tank heater wiring harness from the tank to the main engine harness. On used excavators, check for obvious damage, chafing points where the harness contacts metal brackets, corrosion at connector pins, and rodent damage. Pay special attention to the connector at the DEF tank heater—disconnect it and inspect for moisture, corrosion, or bent pins.

Step 2: Electrical Testing Using a digital multimeter (DVOM), check the DEF heater resistance. Disconnect the heater connector and measure resistance across the heater terminals. Typical resistance should be 2-8 ohms (consult XCMG service manual for exact specifications). An infinite reading indicates an open circuit in the heater element. Next, check for voltage supply at the heater connector with the ignition on—you should see battery voltage (12-14V) when the system commands the heater on.

Step 3: Harness Continuity Check With the heater disconnected from both ends, perform a continuity test on each wire in the harness from the ECM connector to the heater connector. Check for less than 5 ohms resistance on power and ground circuits. Also perform an insulation resistance test between each circuit wire and ground to identify any short circuits (should read infinite resistance).

Step 4: Component Replacement and Verification If the heater element tests faulty, replace the DEF heater assembly. For used excavators, always replace the connector pigtail if any corrosion is present—don't attempt to clean corroded pins. After repairs, clear the fault code using XCMG diagnostic software or a compatible J1939 scan tool, then perform a functionality test by monitoring heater operation in live data while the engine runs.

Disclaimer: This guide provides general diagnostic information. Always consult the official XCMG service manual for your specific machine serial number and software calibration. If you're uncomfortable performing electrical diagnostics, consult a certified XCMG technician or qualified heavy equipment diesel mechanic to prevent further damage to the SCR system.