What is Sany SY245/265H Fault Code 128?

Fault Code 128 on the Sany SY245/265H excavator indicates a malfunction in the Engine Coolant Temperature (ECT) Sensor Circuit, specifically detecting an abnormally high voltage signal or open circuit condition. This code is generated by the Mitsubishi Heavy Industries engine's Electronic Control Module (ECM) when the coolant temperature sensor readings fall outside acceptable parameters (typically above 4.8V or below 0.2V).

This fault is critical for the SY245/265H because the ECT sensor provides essential data for fuel injection timing, idle speed control, and engine protection systems. When this circuit malfunctions, the ECM defaults to failsafe mode, which can significantly impact machine performance, fuel efficiency, and may trigger automatic derate protocols to prevent engine damage. For used excavators, this code often signals deteriorating wiring harnesses or sensor degradation from years of thermal cycling and vibration exposure.

Common Symptoms

• Check Engine Light illuminated on the instrument cluster with possible engine derate mode activated
• Engine running in "limp mode" with reduced power output and limited RPM range (typically restricted to 1500-1800 RPM)
• Difficult cold starts or extended cranking times, especially in cooler ambient temperatures
• Rough idle or hunting/surging at low engine speeds due to improper fuel mixture calculations
• Cooling fan running continuously at high speed regardless of actual engine temperature

Potential Causes

The most common technical failures triggering Code 128 on used Sany SY245/265H excavators include:

• ECT sensor internal failure – sensor element degradation causing erratic resistance values (common after 5,000+ operating hours)
• Wiring harness damage at known rub points near the engine block mounting bracket or along the valve cover edge where cable ties deteriorate
• Corroded or loose connector pins at the two-pin ECT sensor plug, often caused by coolant seepage or moisture intrusion
• Open circuit in the signal wire between the sensor and ECM, frequently occurring where harnesses pass through bulkhead grommets
• ECM ground circuit issues affecting multiple sensor readings simultaneously

How to Troubleshoot and Fix Code 128

Step 1: Visual Inspection Begin with the engine at operating temperature (safety first—allow cooling). Locate the ECT sensor (typically installed in the thermostat housing or cylinder head). Inspect the sensor connector for corrosion, bent pins, or coolant residue. Check the entire wiring harness route for chafing, particularly where it contacts metal edges or passes near hot exhaust components—a common failure point in used machines.

Step 2: Electrical Testing Disconnect the ECT sensor connector. Using a digital multimeter, measure resistance across the sensor terminals. At 68°F (20°C), resistance should read approximately 2,500-3,000 ohms; at 176°F (80°C), approximately 300-400 ohms (consult Mitsubishi specifications for exact values). If readings are infinite (open) or zero (shorted), replace the sensor. Next, check signal wire continuity from the sensor connector to the ECM harness connector—resistance should be less than 1 ohm.

Step 3: Voltage and Circuit Verification With the ignition on but engine off, measure voltage at the ECT sensor connector (harness side). You should see approximately 5V reference voltage from the ECM. If voltage is absent, trace the reference wire for breaks. After repairs, clear codes using Mitsubishi MUT-III diagnostic software or compatible scan tool, run the engine through temperature cycles, and verify the code doesn't return.

For used excavators specifically: Always inspect coolant condition—contaminated coolant accelerates sensor failure. Check for previous repair attempts like spliced wires or aftermarket sensors that may not meet OEM specifications.

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Disclaimer: This guide provides general diagnostic information. Complex electrical issues or ECM-related problems should be evaluated by certified Sany or Mitsubishi Heavy Industries technicians with proper diagnostic equipment. Always consult the manufacturer's service manual for model-specific procedures.