What is Hitachi ZH200LC-5A Fault Code 105-4?

Fault Code 105-4 on the Hitachi ZH200LC-5A excavator indicates an abnormal voltage condition in the Engine Coolant Temperature (ECT) sensor circuit, specifically a "voltage too high" or open circuit failure. This code is generated when the engine control module (ECM) detects a voltage reading above the normal operating range (typically above 4.8V) from the ECT sensor, which monitors coolant temperature to optimize engine performance and prevent overheating.

This fault is critical for the ZH200LC-5A's Isuzu engine because the ECM relies on accurate coolant temperature data to control fuel injection timing, idle speed, and cooling fan operation. When this code activates, the ECM defaults to a predetermined temperature value, which can lead to poor fuel economy, hard starting in cold conditions, and potential engine protection mode activation.

Common Symptoms

• Check engine light illuminated on the instrument panel, often accompanied by a warning buzzer
• Difficulty starting when the engine is cold, or prolonged cranking time
• Cooling fan running continuously at high speed, even when the engine is cold
• Reduced engine power or derate mode activation to protect the engine from perceived overheating
• Black smoke from exhaust due to improper fuel mixture compensation

Potential Causes

The most common causes of Code 105-4 on used ZH200LC-5A excavators include:

• Open circuit or broken wire in the ECT sensor harness, particularly at flex points near the engine mount where vibration causes wire fatigue
• Failed ECT sensor with internal open circuit (common on machines with 5,000+ operating hours)
• Corroded or loose connector pins at the sensor or ECM connection points due to coolant leaks or moisture intrusion
• Damaged ECM wiring harness from rodent activity or previous repair work in the engine compartment
• Faulty ECM (less common, but possible in older machines with electrical system history)

How to Troubleshoot and Fix Code 105-4

Step 1: Visual Inspection Begin by inspecting the ECT sensor located on the engine block near the thermostat housing. Check for coolant leaks, corrosion on the connector, and physical damage to the sensor body. On used excavators, pay special attention to harness wear where wires contact engine components or pass through grommets.

Step 2: Connector and Continuity Testing Disconnect the ECT sensor connector and inspect for bent pins, moisture, or corrosion. Using a digital multimeter, test the sensor resistance (should read approximately 2,000-3,000 ohms at room temperature for most Isuzu engines). Check wire continuity from the sensor connector to the ECM connector pins—any reading of infinite resistance indicates an open circuit.

Step 3: Voltage Testing With the ignition on and sensor disconnected, measure voltage at the ECM supply wire (typically the signal wire). You should see approximately 5V reference voltage from the ECM. If voltage reads above 4.9V constantly, the issue is likely an open sensor ground or signal wire.

Step 4: Sensor Replacement and Harness Repair If testing confirms sensor failure, replace the ECT sensor with an OEM or quality aftermarket part (Hitachi part number varies by engine serial). For harness issues common in used machines, repair damaged wires using heat-shrink solder connections and reroute away from hot or moving components. Clear the fault code using Hitachi diagnostic software (Dr.EX) or a compatible scan tool.

Disclaimer: This guide provides general troubleshooting steps. Always consult the official Hitachi ZH200LC-5A service manual and consider professional diagnostic assistance for complex electrical issues, especially on high-hour used equipment.