What is Sany SY245/265H Fault Code 79?

Fault Code 79 on the Sany SY245/265H excavator indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading detected by the Mitsubishi Heavy Industries engine control module (ECM). This code typically triggers when the ECM receives voltage signals outside the expected range from the coolant temperature sensor (CTS), or when the sensor detects coolant temperatures exceeding safe operating parameters.

The Mitsubishi engine used in these Sany models relies heavily on accurate coolant temperature data to optimize fuel injection timing, control engine speed, and prevent overheating damage. When Code 79 activates, the ECM may enter a protective derate mode, limiting engine power to prevent catastrophic failure. For used excavators, this code often points to sensor degradation or wiring issues common in machines with 5,000+ operating hours.

Common Symptoms

• Engine warning light illuminated on the instrument panel, often amber or red depending on severity
• Reduced engine power or inability to reach full RPM due to ECM-imposed derate protection
• Erratic temperature gauge readings that fluctuate rapidly or remain stuck at maximum/minimum
• Difficulty starting when the engine is cold, as the ECM receives incorrect temperature data for fuel mixture calculations
• Activation of cooling fan at inappropriate times, running continuously even when engine is cool

Potential Causes

The most frequent causes of Code 79 in used Sany SY245/265H excavators include:

• Faulty coolant temperature sensor due to internal element degradation or contamination from coolant additives
• Damaged or corroded wiring harness near the sensor connector, especially at friction points where harnesses contact the engine block
• Poor electrical connections at the CTS connector caused by moisture intrusion, corrosion, or loose terminal pins
• Low coolant level exposing the sensor element to air instead of liquid coolant
• ECM internal failure (rare) where the temperature input circuit has failed
• Aftermarket coolant incompatible with sensor materials, causing accelerated sensor failure in older machines

How to Troubleshoot and Fix Code 79

Step 1: Visual Inspection and Coolant Check Begin by verifying the coolant level in the radiator and overflow tank. Low coolant can expose the sensor, causing erratic readings. Inspect the coolant temperature sensor location (typically on the engine block or thermostat housing) for physical damage, coolant leaks, or corrosion on the connector terminals. For used excavators, pay special attention to wiring harness routing—look for abrasion points where harnesses contact metal surfaces or moving components.

Step 2: Electrical Testing with Multimeter Disconnect the CTS electrical connector and measure resistance across the sensor terminals using a digital multimeter. A typical Mitsubishi engine CTS shows approximately 2,000-3,000 ohms at room temperature (20°C/68°F) and decreases to 200-400 ohms at operating temperature (80-90°C/176-194°F). Compare readings against manufacturer specifications. Also check for voltage supply from the ECM—you should see approximately 5 volts on the signal wire with the key on, engine off.

Step 3: Harness and Connector Repair Inspect the entire wiring harness from sensor to ECM for damaged insulation, particularly in high-vibration areas. On used machines, connector corrosion is extremely common. Clean connector pins with electrical contact cleaner and apply dielectric grease to prevent future moisture intrusion. Check for proper ground continuity on the sensor ground circuit.

Step 4: Sensor Replacement and Code Clearing If testing confirms sensor failure, replace with a genuine Mitsubishi or OEM-equivalent coolant temperature sensor. After installation, use Mitsubishi MUT-III diagnostic software or compatible scan tool to clear the fault code and monitor live coolant temperature data during a warm-up cycle to verify proper operation.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Sany service manual and Mitsubishi engine documentation for your specific machine serial number. If you lack proper diagnostic tools or experience with high-voltage electrical systems, consult a certified heavy equipment technician to prevent injury or further damage.