Hitachi ZX350LCK-5G Fault Code 20105-2: Technical Diagnosis and Repair Guide

What is Hitachi ZX350LCK-5G Fault Code 20105-2?

Fault Code 20105-2 indicates a malfunction in the engine coolant temperature sensor circuit, specifically a low voltage condition detected by the Engine Control Module (ECM). This code triggers when the ECM receives voltage readings below the manufacturer's specified threshold, typically under 0.5 volts, from the coolant temperature sensor signal wire.

This fault is critical for the ZX350LCK-5G because the ECM relies on accurate coolant temperature data to manage fuel injection timing, idle speed control, and engine protection protocols. Without reliable temperature readings, the machine may enter derate mode or experience poor cold-start performance, directly impacting productivity on job sites.

Common Symptoms

When Code 20105-2 is active, operators typically experience:

• Check Engine Light illuminated on the instrument panel with possible temperature gauge malfunction
• Hard starting conditions, especially during cold weather, due to improper fuel enrichment
• Rough idling or hunting engine RPM as the ECM defaults to fallback temperature values
• Reduced engine power or activation of engine derate mode to prevent potential overheating damage
• Cooling fan running continuously at high speed regardless of actual engine temperature

Potential Causes

The most common technical failures triggering this code on used ZX350LCK-5G excavators include:

• Coolant temperature sensor failure – internal sensor resistance deterioration common after 5,000+ operating hours
• Damaged or corroded wiring harness – particularly at the sensor connector near the thermostat housing where heat cycling causes connector pin corrosion
• Shorted signal wire to ground – common rub-through point where the harness passes near the engine block mounting bracket
• Failed ECM internal circuitry – rare but possible on high-hour machines with moisture intrusion history
• Poor ground connections at engine block grounding points due to corrosion buildup

How to Troubleshoot and Fix Code 20105-2

Step 1: Visual Inspection Inspect the coolant temperature sensor connector located near the thermostat housing. On used excavators, check for corrosion, moisture intrusion, or damaged connector pins. Examine the wiring harness for chafing points against metal surfaces – the ZX350LCK-5G has a known rub point near the fuel filter bracket.

Step 2: Sensor Resistance Testing Disconnect the sensor and measure resistance across its terminals using a digital multimeter. At 20°C (68°F), resistance should read approximately 2,000-3,000 ohms. Compare readings against Hitachi's temperature-resistance chart. Replace the sensor if readings are infinite (open circuit) or near zero ohms.

Step 3: Circuit Voltage Testing With the ignition on and sensor disconnected, check signal wire voltage at the harness connector. It should read approximately 5 volts (reference voltage from ECM). Low or zero voltage indicates wiring issues or ECM problems. Perform continuity tests between the sensor connector and ECM pins, checking for shorts to ground.

Step 4: Harness and Ground Verification Inspect all engine ground straps for tightness and corrosion, particularly the ground between the engine block and chassis frame. Clean and restore ground connections before replacing expensive components.

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Disclaimer: This guide provides general diagnostic procedures. Always consult the official Hitachi service manual for your specific machine serial number and consider professional diagnostic assistance for complex electrical issues.