What is XCMG XE750D Fault Code 171-4 (SPN171 FMI4)?

Fault Code 171-4 on the XCMG XE750D excavator indicates a voltage problem with the Engine Coolant Temperature (ECT) sensor circuit, specifically reporting "Voltage Below Normal or Shorted to Low Source." This code follows the J1939 standard where SPN 171 refers to the engine coolant temperature sensor, and FMI 4 designates a circuit voltage that has fallen below acceptable parameters or is shorted to ground.

This fault is critical for the XE750D's engine management system because the Electronic Control Module (ECM) relies on accurate coolant temperature data to optimize fuel injection timing, control idle speed, and manage the engine's cooling fan operation. When this sensor circuit malfunctions, the ECM operates in default mode, which can severely impact performance and fuel efficiency on this large-class excavator.

Common Symptoms

• Check Engine Light (CEL) or malfunction indicator lamp illuminated on the instrument cluster
• Engine derate or reduced power output, particularly noticeable during heavy digging or lifting operations
• Difficulty starting when the engine is cold, or extended cranking times
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Rough idle or irregular engine RPM fluctuations during warm-up periods

Potential Causes

The most common technical causes for Code 171-4 on used XCMG XE750D excavators include:

• Damaged or corroded ECT sensor connector – particularly vulnerable to moisture intrusion in the engine compartment
• Chafed or broken wiring harness between the ECT sensor and ECM, often caused by rubbing against engine components or frame rails
• Failed ECT sensor with internal short circuit or degraded thermistor element (common in machines with 5,000+ operating hours)
• Corroded or loose ground connections on the engine block affecting sensor circuit integrity
• ECM internal fault (rare, but possible in older units that have experienced electrical surges)

How to Troubleshoot and Fix Code 171-4

Step 1: Visual Inspection Begin by locating the ECT sensor on the engine block (typically near the thermostat housing). Inspect the sensor connector for corrosion, bent pins, or moisture contamination. On used excavators, pay special attention to connector seal integrity, as environmental exposure over thousands of hours causes deterioration.

Step 2: Electrical Testing Disconnect the ECT sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At room temperature (68°F/20°C), resistance should typically read between 2,000-3,000 ohms (consult XCMG specifications for exact values). Check for continuity between sensor pins and ground – any reading indicates a short circuit requiring sensor replacement.

Step 3: Wiring Harness Verification Trace the sensor harness from the ECT sensor back to the ECM connector. Look for damaged insulation, particularly where the harness passes near hot exhaust components or sharp edges. Use your multimeter to check for continuity and proper resistance (typically under 5 ohms) along the signal and ground wires.

Step 4: Voltage Supply Check With the ignition on but engine off, back-probe the ECT sensor connector to measure reference voltage from the ECM. You should see approximately 5 volts DC on the signal wire. Voltage significantly below this indicates ECM issues or harness problems.

Step 5: Clear Code and Monitor After repairs, clear the fault code using XCMG diagnostic software or a compatible J1939 scan tool. Operate the excavator through a complete heating cycle and monitor for code recurrence.

Disclaimer: This guide provides general troubleshooting information. Always consult the XCMG XE750D service manual for model-specific procedures and specifications. For complex electrical diagnostics or ECM programming, seek assistance from certified XCMG technicians or qualified heavy equipment diesel mechanics.