What is Hitachi ZX350K-5G Fault Code 20105-2?

Fault Code 20105-2 on the Hitachi ZX350K-5G excavator indicates an abnormal voltage signal from the Engine Coolant Temperature (ECT) sensor circuit, specifically a high voltage condition or open circuit detected by the Engine Control Module (ECM). This diagnostic trouble code (DTC) signifies that the ECM is receiving a voltage reading outside the expected operational range, typically above 4.5 volts, when normal operating conditions should produce 0.5-3.5 volts depending on coolant temperature.

This code is critical for the ZX350K-5G's Isuzu engine management system because accurate coolant temperature data directly influences fuel injection timing, idle speed control, and engine protection protocols. When the ECM cannot reliably read coolant temperature, it may default to conservative operating parameters that reduce machine performance and fuel efficiency.

Common Symptoms

• Engine warning light illuminated on the instrument cluster with possible audible alarm
• Reduced engine power or derate mode activated as a protective measure
• Hard starting conditions, especially in cold weather, due to incorrect fuel enrichment
• Erratic idle speed or rough running as the ECM compensates for missing temperature data
• Cooling fan running continuously at high speed regardless of actual engine temperature

Potential Causes

The most frequent causes of Code 20105-2 on used ZX350K-5G excavators include:

• Failed ECT sensor with internal open circuit or degraded resistance element (common after 5,000+ operating hours)
• Damaged wiring harness between the sensor and ECM, particularly at flex points near the engine mount or along the frame rail where vibration causes wire chafing
• Corroded or loose connector pins at the ECT sensor plug or ECM harness connector (moisture intrusion is common in the engine compartment)
• Broken or stretched sensor retaining clip allowing coolant leakage and sensor displacement
• ECM internal fault (rare, but possible in machines with electrical system history or previous water damage)

How to Troubleshoot and Fix Code 20105-2

Step 1: Visual Inspection Begin with the engine cool. Locate the ECT sensor on the engine block (typically near the thermostat housing on Isuzu engines). Inspect the sensor connector for corrosion, pushed-back pins, or moisture. Check the wiring harness for obvious damage, particularly where it routes near moving components or sharp edges common in used excavators.

Step 2: Electrical Testing Disconnect the ECT sensor connector. Using a digital multimeter, measure resistance across the sensor terminals. At approximately 68°F (20°C), resistance should be 2,000-3,000 ohms. Compare your reading to the manufacturer's temperature-resistance chart. Next, check for continuity in the wiring between the sensor connector and ECM pins (consult wiring diagrams for specific pin locations). Resistance should be less than 5 ohms with no shorts to ground.

Step 3: Voltage Supply 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 or incorrect, trace the circuit back toward the ECM, checking for damaged wires or poor ground connections at the engine block grounding points.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Hitachi or OEM-equivalent ECT sensor using proper thread sealant. For used excavators, always replace the connector pigtail if corrosion is present—this prevents recurring codes. After installation, clear codes using Dr.EX diagnostic software or compatible scan tool and verify proper operation through a complete heat cycle.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Hitachi service manual for your specific machine serial number and consider professional diagnostic assistance for complex electrical issues or if you lack proper diagnostic equipment.