Hitachi EX350LC-5 HHE Fault Code 04: Complete Diagnostic Guide

What is Hitachi EX350LC-5 HHE Fault Code 04?

Fault Code 04 on the Hitachi EX350LC-5 HHE indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading detected by the Engine Control Module (ECM). This code specifically relates to the temperature monitoring system that protects the Isuzu engine from overheating damage.

The coolant temperature sensor provides critical data to the ECM, which uses this information to adjust fuel injection timing, control engine speed, and activate warning systems. On used excavators like the EX350LC-5 HHE, this sensor's accuracy is essential for preventing catastrophic engine damage and maintaining optimal hydraulic system performance, as engine temperature directly affects hydraulic oil viscosity and system efficiency.

Common Symptoms

When Code 04 is active, operators typically experience:

• Engine warning light illuminated on the instrument panel, often accompanied by an audible alarm
• Reduced engine power or automatic derate mode engaging to protect the engine
• Erratic temperature gauge readings that fluctuate abnormally or show maximum temperature immediately at startup
• Difficulty starting the engine, particularly in cold weather conditions
• Poor fuel economy and black smoke from the exhaust due to incorrect fuel mapping

Potential Causes

The most common technical reasons for Code 04 on used EX350LC-5 HHE excavators include:

• Faulty coolant temperature sensor due to internal element degradation (common after 5,000+ operating hours)
• Damaged or corroded wiring harness at connector points, especially near the engine block where heat and vibration cause wear
• Corroded sensor connector pins from coolant leaks or moisture intrusion in the engine bay
• Low coolant level causing intermittent sensor exposure and false readings
• ECM internal fault or corrupted calibration data (less common but possible in high-hour machines)
• Broken or chafed wires at known rub points between the sensor and ECM, particularly where the harness passes near the engine mount

How to Troubleshoot and Fix Code 04

Step 1: Visual Inspection Begin by checking the coolant level in the radiator and overflow tank. Inspect the coolant temperature sensor (typically located on the engine block near the thermostat housing) for physical damage, coolant leaks, or corrosion. On used excavators, carefully examine the wiring harness for signs of rubbing, cracking, or heat damage along its entire length.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor connector and measure the sensor resistance. At 20°C (68°F), resistance should typically read 2,000-3,000 ohms (consult your service manual for exact specifications). Check for continuity in the wiring between the sensor and ECM connector pins. Measure supply voltage from the ECM (should be approximately 5 volts).

Step 3: Component Replacement and Verification If resistance values are out of specification or wiring shows damage, replace the faulty component. For used machines, always clean connector terminals with electrical contact cleaner and apply dielectric grease to prevent future corrosion. After repairs, clear the fault code using Hitachi Dr.EX diagnostic software or a compatible scan tool, then monitor the system during a complete heat cycle.

Disclaimer: This guide provides general diagnostic information. Always consult the official Hitachi service manual for your specific machine serial number and consider professional assistance for complex electrical diagnostics or if you're unfamiliar with high-voltage systems.