What is XCMG XE750D Fault Code 444-6 SPN677 FMI6?

Fault Code 444-6 (SPN677 FMI6) indicates a current below normal or open circuit condition in the Engine Coolant Temperature (ECT) sensor circuit. This code is triggered when the Electronic Control Module (ECM) detects abnormally high resistance or an open circuit in the coolant temperature sensor wiring or sensor itself.

On the XCMG XE750D excavator, the ECT sensor is critical for engine performance management. It monitors coolant temperature and sends voltage signals to the ECM, which adjusts fuel injection timing, idle speed, and cooling fan operation. When this circuit fails, the ECM cannot accurately manage engine temperature, potentially leading to overheating, poor fuel economy, or engine protection mode activation.

Common Symptoms

• Check Engine Light or engine diagnostic warning illuminated on the dashboard
• Engine operating in derate mode (reduced power output) to prevent potential damage
• Cooling fan running continuously at high speed, regardless of actual engine temperature
• Difficulty starting the engine, especially in cold conditions
• Abnormal idle speed (too high or too low) due to incorrect temperature compensation
• ECM defaulting to failsafe temperature values, which may cause rough running or black smoke

Potential Causes

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

• Open circuit or broken wiring in the ECT sensor harness, particularly at flex points near the engine block where vibration causes wire fatigue
• Corroded or loose connector pins at the ECT sensor plug—common in machines exposed to moisture or coolant leaks
• Failed ECT sensor itself due to internal element failure or physical damage from thermal cycling
• Harness chafing against engine components, especially where protective looms have deteriorated over time
• Poor ground connection at the sensor circuit return path
• Previous repair attempts using incorrect connector sealing, allowing moisture ingress in used equipment

How to Troubleshoot and Fix Code 444-6 SPN677 FMI6

Step 1: Visual Inspection Begin by locating the ECT sensor on the engine block (typically near the thermostat housing). Inspect the wiring harness for visible damage, chafing, or corrosion. On used excavators, check for coolant residue around the sensor indicating leaks that may have damaged connectors.

Step 2: Connector and Pin Testing Disconnect the ECT sensor connector and inspect pins for corrosion, bent terminals, or moisture. Clean with electrical contact cleaner and verify proper pin tension. Check connector seals for deterioration.

Step 3: Resistance Testing Using a digital multimeter, measure resistance across the ECT sensor terminals. Compare readings against manufacturer specifications (typically 2,000-3,000 ohms at 20°C). Readings showing infinite resistance confirm sensor or circuit failure.

Step 4: Circuit Continuity Check Test continuity between the sensor connector and ECM pins using wiring diagrams. Check for open circuits in the harness—critical in used machines where wire insulation may have degraded.

Step 5: Voltage Supply Verification With ignition on, measure reference voltage at the sensor connector (should be approximately 5 volts). No voltage indicates ECM or supply circuit issues.

Step 6: Component Replacement Replace the ECT sensor if testing confirms failure. For used excavators, always replace the connector pigtail if corrosion is present to prevent recurring failures. Use dielectric grease on connections.

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Disclaimer: This guide provides general diagnostic information. Always consult the XCMG service manual for your specific machine serial number and consider professional diagnostic assistance for complex electrical issues or if you lack proper testing equipment.