What is Sany SY245/265H Fault Code 120?

Fault Code 120 on the Sany SY245/265H excavator indicates an Engine Coolant Temperature (ECT) Sensor Circuit High Voltage condition. This diagnostic trouble code is generated by the Mitsubishi Heavy Industries engine control module (ECM) when it detects an abnormally high voltage signal from the coolant temperature sensor circuit, typically above 4.5-4.8 volts.

The ECT sensor is a critical component that monitors engine operating temperature and sends real-time data to the ECM. This information directly influences fuel injection timing, idle speed control, and engine protection protocols. When the ECM receives a voltage reading outside normal parameters (usually 0.5-4.5V), it cannot accurately determine coolant temperature, potentially leading to improper engine management and possible overheating damage on these Mitsubishi-powered excavators.

Common Symptoms

• Check Engine Light or malfunction indicator lamp illuminated on the instrument cluster
• Engine running in cold-start mode continuously, causing rough idle and black smoke due to rich fuel mixture
• High idle RPM that doesn't decrease as engine warms up
• Poor fuel economy and increased diesel consumption during operation
• Potential engine derate or power reduction in some cases to protect components

Potential Causes

The most common technical failures triggering Code 120 on used Sany excavators include:

• Open circuit in the ECT sensor wiring harness, often caused by wire breakage near engine mounting points where vibration stress is highest
• Damaged or corroded connector at the ECT sensor itself—a frequent issue on machines with 3,000+ operating hours
• Failed ECT sensor with internal open circuit (less common than wiring issues)
• Harness chafing against engine components, particularly near the thermostat housing where the sensor is mounted
• ECM-side connector corrosion or damaged pins at the engine control module

How to Troubleshoot and Fix Code 120

Step 1: Visual Inspection Locate the ECT sensor on the engine block (typically near the thermostat housing on Mitsubishi engines). Inspect the sensor connector and wiring harness for obvious damage, corrosion, oil contamination, or loose connections. On used excavators, pay special attention to harness routing where it contacts hot engine surfaces.

Step 2: Electrical Testing Disconnect the ECT sensor connector. Using a digital multimeter, measure resistance across the sensor terminals. At room temperature (68°F/20°C), resistance should read approximately 2,000-3,000 ohms depending on specifications. Also check for continuity between sensor terminals and ground—there should be infinite resistance (open circuit).

Step 3: Circuit Voltage Check With the sensor disconnected and ignition ON (engine off), measure voltage at the ECM-side harness connector. You should see approximately 5 volts reference voltage from the ECM. If voltage reads battery voltage (12-24V) or fluctuates erratically, suspect wiring short to power or ECM issues.

Step 4: Repair or Replace If sensor resistance is out of range, replace the ECT sensor with OEM or quality aftermarket parts. If wiring shows damage, repair using heat-shrink solder connections and reroute away from heat/friction points. Clear codes using Mitsubishi diagnostic software or compatible scan tool and verify repair.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Sany service manual and consider professional diagnostic services for complex electrical issues or if unfamiliar with high-voltage systems.