What is Sany SY245/265H Fault Code 78?

Fault Code 78 on the Sany SY245/265H excavator indicates a malfunction in the Engine Coolant Temperature (ECT) Sensor Circuit within the Mitsubishi Heavy Industries diesel engine system. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormal voltage signal from the coolant temperature sensor, typically indicating an open circuit, short circuit, or sensor reading outside acceptable parameters (usually -40°F to 300°F/-40°C to 150°C).

The ECT sensor plays a critical role in engine management by monitoring coolant temperature and sending real-time data to the ECM. This information directly affects fuel injection timing, idle speed control, and emission system operation. In Mitsubishi diesel engines powering these Sany models, accurate coolant temperature readings are essential for preventing overheating, optimizing combustion efficiency, and protecting expensive engine components from thermal damage.

Common Symptoms

When Code 78 is active on your SY245/265H, operators typically experience:

• Engine warning light illuminated on the instrument cluster, often accompanied by audible alarms
• Hard starting conditions, especially during cold weather, as the ECM cannot properly adjust fuel delivery
• Poor idle quality or hunting/surging RPM due to incorrect fuel mixture calculations
• Reduced engine performance or activation of derate mode limiting machine power output
• Cooling fan running continuously at high speed regardless of actual engine temperature

Potential Causes

The most common technical reasons for Code 78 appearing on used Sany excavators include:

• Failed ECT sensor due to internal element deterioration or contamination from coolant additives (common after 5,000+ operating hours)
• Damaged wiring harness at known rub points near the engine block or where the harness routes over the valve cover
• Corroded or loose connector terminals at the sensor plug, especially on machines exposed to harsh environments or poor maintenance
• Broken or chafed wiring between the ECT sensor and ECM, particularly in the engine bay where heat cycling causes insulation breakdown
• ECM internal fault (less common) affecting the sensor input circuit or ground reference

How to Troubleshoot and Fix Code 78

Step 1: Visual Inspection Begin by locating the ECT sensor (typically installed in the engine coolant outlet housing near the thermostat). Inspect the sensor connector for corrosion, moisture intrusion, bent pins, or loose connections. On used excavators, check the wiring harness routing for obvious damage, wear marks from rubbing, or heat damage from exhaust components.

Step 2: Electrical Testing Disconnect the ECT sensor connector and use a digital multimeter to measure sensor resistance. At 68°F (20°C), resistance should typically read 2,000-3,000 ohms (consult Mitsubishi specifications for exact values). Test harness continuity from the sensor connector back to the ECM pins, checking for opens or shorts to ground. Measure reference voltage at the harness connector with ignition on—expect approximately 5 volts from the ECM.

Step 3: Component Replacement and Verification If sensor resistance is out of specification or harness testing reveals wiring faults, replace the defective component using genuine Mitsubishi or OEM-equivalent parts. For used machines, always apply dielectric grease to connector terminals to prevent future corrosion. Clear fault codes using Mitsubishi MUT-III diagnostic software or compatible scan tool, then perform a test cycle monitoring live coolant temperature data to confirm proper operation.

Critical for used excavators: Before replacing the ECT sensor, thoroughly inspect coolant condition—contaminated or degraded coolant can cause premature sensor failure and repeat codes.

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Disclaimer: This guide provides general troubleshooting information. Always consult the manufacturer's service manual and consider professional diagnostic assistance for complex electrical issues or if you lack proper testing equipment.