Hitachi EX200-3 Fault Code 35: Complete Diagnostic Guide

What is Hitachi EX200-3 Fault Code 35?

Fault Code 35 on the Hitachi EX200-3 excavator indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading. This code is triggered when the Electronic Control Module (ECM) detects voltage signals outside the normal operating range from the coolant temperature sensor, typically between 0.5-4.5 volts.

The coolant temperature sensor is critical for the EX200-3's Isuzu 6BG1 engine management system. It directly influences fuel injection timing, idle speed control, and engine protection protocols. When this sensor fails or provides incorrect data, the ECM cannot properly regulate engine performance, potentially leading to overheating, poor fuel economy, or engine damage.

Common Symptoms

When Code 35 is active, operators typically experience:

• Engine warning light illuminated on the instrument panel
• Hard starting or extended cranking time, especially in cold conditions
• Rough idle or unstable engine RPM during warm-up
• Black smoke from the exhaust due to improper fuel mixture
• Engine overheating or cooling fan running continuously, even when engine is cold

Potential Causes

The most common technical reasons for Code 35 on used EX200-3 excavators include:

• Coolant temperature sensor failure due to age-related degradation or internal short circuits
• Damaged wiring harness near the engine block where vibration and heat cause insulation breakdown
• Corroded or loose connector pins at the sensor plug, common in machines with 5,000+ operating hours
• ECM internal fault affecting the sensor input circuit (rare but possible in high-hour machines)
• Coolant contamination causing sensor element coating and inaccurate readings
• Ground circuit failure between sensor and ECM chassis ground

How to Troubleshoot and Fix Code 35

Step 1: Visual Inspection Begin by locating the coolant temperature sensor on the engine block (typically near the thermostat housing). Inspect the wiring harness for obvious damage, chafing against the engine block, or melted insulation. Check the connector for corrosion, bent pins, or coolant intrusion—a common issue on used excavators operating in harsh environments.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure resistance across its terminals. At 68°F (20°C), resistance should read approximately 2,000-3,000 ohms. Compare readings against Hitachi specifications. Next, check voltage supply at the harness connector with ignition on—you should see approximately 5 volts reference voltage from the ECM.

Step 3: Circuit and Ground Verification Test continuity between the sensor connector and ECM pins (consult wiring diagrams). Check for short circuits to ground or power. Verify the ground circuit has less than 0.5 ohms resistance. On used machines, corrosion at ground points is frequently overlooked.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Hitachi or OEM-equivalent sensor. Before installation, clean the mounting bore and apply anti-seize compound. Clear codes using Hitachi diagnostic software (Dr.EX) or a compatible scan tool, then monitor for code recurrence.

Disclaimer: This guide provides general diagnostic information. Always consult the official Hitachi service manual for your specific machine serial number. If you're uncomfortable performing electrical diagnostics, seek assistance from a qualified heavy equipment technician to prevent further damage.