What is Hitachi EX200-3 Fault Code 108-4?

Fault Code 108-4 on the Hitachi EX200-3 excavator indicates a malfunction in the engine coolant temperature sensor circuit, specifically a voltage signal that is too high or out of normal operating range. This diagnostic trouble code (DTC) is generated when the Electronic Control Module (ECM) detects an abnormally high voltage reading from the coolant temperature sensor, typically above 4.5 volts, suggesting an open circuit or sensor failure.

This code is critical for the EX200-3's performance because the engine coolant temperature directly influences fuel injection timing, idle speed control, and engine protection systems. When the ECM cannot accurately read coolant temperature, it defaults to a pre-programmed value that may cause poor fuel economy, rough idling, or premature engine component wear on these older machines.

Common Symptoms

When Code 108-4 is active on your Hitachi EX200-3, you may experience:

• Check engine warning light illuminated on the instrument cluster
• Hard starting, especially in cold weather conditions, as the ECM cannot properly adjust fuel enrichment
• Rough idle or erratic engine RPM during warm-up cycles
• Black smoke from exhaust due to incorrect fuel-to-air mixture calculations
• Engine overheating without proper warning, as the temperature monitoring system is compromised

Potential Causes

The most common technical reasons for Code 108-4 on used EX200-3 excavators include:

• Failed coolant temperature sensor – internal resistance breakdown is common after 5,000+ operating hours
• Open circuit in sensor wiring harness – particularly at flex points near the engine mount where vibration causes wire fatigue
• Corroded or loose connector pins at the sensor or ECM connection points (common in machines exposed to moisture)
• Damaged sensor wiring from contact with hot engine components or rubbing against sharp edges
• ECM internal circuit failure (rare, but possible in high-hour machines)
• Poor ground connection at the engine block or ECM chassis ground

How to Troubleshoot and Fix Code 108-4

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the engine block (typically near the thermostat housing). Check for physical damage, coolant leaks around the sensor, or corroded connector terminals. On used excavators, examine the entire wiring harness from sensor to ECM for abraded insulation, especially where it routes near moving components or sharp metal edges.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At room temperature (68°F/20°C), resistance should read approximately 2,000-3,000 ohms; at operating temperature (176°F/80°C), it should drop to 200-400 ohms. If readings are infinite (open circuit) or zero (short circuit), replace the sensor. Also check for supply voltage at the harness connector – it should read approximately 5 volts with ignition on.

Step 3: Wiring and Ground Verification Test continuity from the sensor connector back to the ECM pins using wiring diagrams. Check for voltage drops exceeding 0.2 volts, indicating high resistance from corrosion or damaged wires. Inspect and clean all ground connections at the engine block and ECM mounting points, as poor grounds frequently cause high voltage fault codes on older machines.

Step 4: Component Replacement and Verification If testing confirms sensor failure, replace with an OEM or quality aftermarket coolant temperature sensor and apply dielectric grease to connector terminals. After replacement, clear the fault code using diagnostic software compatible with Hitachi equipment, run the engine through a complete warm-up cycle, and verify the code does not return.

Disclaimer: This guide provides general troubleshooting information for the Hitachi EX200-3. Always consult the manufacturer's service manual for your specific machine serial number and consider professional diagnostic assistance for complex electrical issues or if you're unfamiliar with high-voltage systems.