What is Sany SY245/265H Fault Code 125?

Fault Code 125 indicates a malfunction in the Engine Coolant Temperature (ECT) sensor circuit, specifically detecting an abnormally high voltage signal in the Mitsubishi Heavy Industries engine control system. This diagnostic trouble code (DTC) signals that the Engine Control Module (ECM) is receiving voltage readings outside the expected range from the coolant temperature sensor, typically above 4.5 volts when normal operating range should be 0.5-4.0 volts.

This fault is critical for the Sany SY245/265H because the ECT sensor provides essential data for fuel injection timing, idle speed control, and engine protection systems. When the ECM cannot accurately monitor coolant temperature, it may default to fail-safe mode, limiting machine performance to prevent potential engine damage from overheating conditions.

Common Symptoms

• Check Engine Light or warning lamp illuminated on the instrument cluster
• Engine running in limp mode with reduced power output and limited RPM range
• Difficulty starting the engine, particularly in cold weather conditions
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Black smoke from exhaust due to improper fuel mixture compensation

Potential Causes

The most frequent causes of Code 125 in used Sany excavators include:

• Open circuit in the ECT sensor wiring harness, often caused by wire chafing near the engine block mounting points
• Failed coolant temperature sensor with internal circuit breakdown (common after 5,000+ operating hours)
• Corroded or damaged connector pins at the sensor plug, especially on machines exposed to harsh environmental conditions
• ECM internal fault affecting the sensor reference voltage circuit
• Damaged wiring insulation from rodent activity or heat exposure near exhaust components
• Poor ground connection at the engine harness ground points

How to Troubleshoot and Fix Code 125

Step 1: Visual Inspection Begin by inspecting the ECT sensor located on the engine block near the thermostat housing. Check the wiring harness for visible damage, particularly where it routes along the valve cover. On used excavators, examine connector pins for corrosion or bent terminals that could cause intermittent high-resistance connections.

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 read approximately 2,000-3,000 ohms. Next, check for reference voltage at the harness connector with ignition on—you should see approximately 5 volts from the ECM.

Step 3: Circuit Verification Test for continuity between the sensor connector and ECM pins using the excavator's wiring diagram. Check the ground circuit integrity by measuring resistance to chassis ground (should be less than 1 ohm). Use Mitsubishi MUT-III diagnostic software or compatible scan tool to monitor live ECT sensor data while the engine warms up.

Step 4: Component Replacement If testing confirms sensor failure, replace the ECT sensor using OEM or quality aftermarket parts. Apply dielectric grease to connector terminals before reassembly. Clear fault codes and perform a complete warm-up cycle to verify repair.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Sany service manual and consider professional diagnostic assistance for complex electrical issues, especially on used equipment with unknown maintenance history.