Sany SY245/265H Fault Code 123: Complete Diagnostic Guide

What is Sany SY245/265H Fault Code 123?

Fault Code 123 on the Sany SY245/265H indicates a malfunction in the Engine Coolant Temperature (ECT) Sensor circuit. This code is triggered when the Mitsubishi engine's Electronic Control Module (ECM) detects an abnormal voltage signal from the coolant temperature sensor, typically indicating an open circuit, short circuit, or sensor failure.

The ECT sensor plays a critical role in the Mitsubishi Heavy Industries engine management system by monitoring coolant temperature and sending real-time data to the ECM. This information directly affects fuel injection timing, idle speed control, and engine protection protocols. When Code 123 appears, the ECM switches to default temperature values, which can significantly impact engine performance and fuel efficiency on these mid-sized excavators.

Common Symptoms

When Fault Code 123 is active, operators typically experience:

• Check Engine Light illuminated on the instrument panel with reduced engine performance
• Hard starting conditions, especially in cold weather, due to incorrect fuel mixture calculations
• Rough idling or unstable engine RPM at operating temperature
• Increased fuel consumption as the ECM operates in fail-safe mode with default temperature parameters
• Cooling fan running continuously at high speed regardless of actual engine temperature

Potential Causes

The most common technical reasons for Code 123 on used SY245/265H excavators include:

• Failed ECT sensor due to internal element degradation (common after 5,000+ operating hours)
• Corroded or loose electrical connectors at the sensor or ECM harness connection points
• Damaged wiring harness caused by rubbing against the engine block near the thermostat housing (known wear point on this model)
• Coolant contamination on sensor terminals causing false resistance readings
• ECM internal fault (rare, but possible in high-hour machines with electrical system history)

How to Troubleshoot and Fix Code 123

Step 1: Visual Inspection Begin by inspecting the ECT sensor located near the thermostat housing on the Mitsubishi engine. Check for coolant leaks, corrosion on the two-pin connector, and physical damage to the sensor body. On used excavators, carefully examine the wiring harness for abrasion points where it contacts metal surfaces—this is a common failure point.

Step 2: Electrical Testing Disconnect the sensor and use a digital multimeter to measure resistance across the sensor terminals. At 20°C (68°F), resistance should read approximately 2,000-3,000 ohms (consult your service manual for exact specifications). Test the harness continuity from the connector to the ECM to rule out open circuits.

Step 3: Signal Voltage Check With the ignition on and sensor disconnected, measure reference voltage at the harness connector—you should see approximately 5 volts from the ECM. If voltage is absent, trace the harness for breaks or check ECM power supply circuits.

Step 4: Replacement and Verification If the sensor fails testing, replace it with a genuine Mitsubishi or OEM-equivalent ECT sensor. Clean all connector pins with electrical contact cleaner and apply dielectric grease to prevent future corrosion. Clear the fault code using Mitsubishi MUT-III diagnostic software or compatible scan tool, then monitor for code recurrence during a complete heat cycle.

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Disclaimer: This guide provides general diagnostic procedures for Fault Code 123. Always consult the official Sany service manual and consider professional assistance from certified Mitsubishi diesel technicians for complex electrical diagnostics or if you're unfamiliar with high-voltage systems.