Hitachi EX1200-5 Fault Code 06: Complete Diagnostic Guide

What is Hitachi EX1200-5 Fault Code 06?

Fault Code 06 on the Hitachi EX1200-5 excavator indicates an Engine Coolant Temperature (ECT) Sensor Circuit malfunction or abnormal reading. This code triggers when the Engine Control Module (ECM) detects voltage signals outside the normal operating range from the coolant temperature sensor circuit.

The ECT sensor plays a critical role in engine management by monitoring coolant temperature and sending real-time data to the ECM. This information directly affects fuel injection timing, idle speed control, and engine protection protocols. On a large mining-class excavator like the EX1200-5, maintaining proper engine temperature monitoring is essential for preventing catastrophic engine damage and ensuring optimal hydraulic system performance.

Common Symptoms

When Code 06 is active, operators typically experience:

• Check Engine light illuminated on the instrument panel with possible audible warning alarm
• Engine running in default/limp mode with reduced power output and RPM limitations
• Difficulty starting in cold conditions or excessive cranking time
• Erratic engine idle or hunting RPM, particularly during warm-up cycles
• Cooling fan running continuously at high speed regardless of actual engine temperature

Potential Causes

The most common technical reasons for Code 06 on used EX1200-5 excavators include:

• ECT sensor failure due to internal element degradation or contamination from coolant additives
• Damaged or corroded wiring harness connections at the sensor plug or ECM terminals
• Chafed wiring along the engine block where harness routing creates friction points (common wear area near fuel injection pump)
• Coolant contamination causing sensor element coating or corrosion
• ECM internal circuit failure (less common but possible on high-hour machines)
• Incorrect coolant mixture affecting sensor resistance readings
• Previous repair attempts using non-OEM sensors with incompatible resistance curves

How to Troubleshoot and Fix Code 06

Step 1: Visual Inspection Begin by inspecting the ECT sensor location (typically on the engine block near the thermostat housing). Check for coolant leaks, corrosion on the connector, and physical damage. On used excavators, carefully examine the wiring harness for abrasion points where it contacts the engine block or frame—this is a known failure point on EX1200-5 models.

Step 2: Electrical Testing Disconnect the ECT sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At 20°C (68°F), resistance should typically read 2,000-3,000 ohms (consult your service manual for exact specifications). Test harness continuity from the sensor connector to the ECM and check for short circuits to ground.

Step 3: Voltage Check With the ignition on and sensor disconnected, measure reference voltage at the harness connector—should read approximately 5 volts. If voltage is absent or incorrect, trace wiring back to the ECM for breaks or shorts.

Step 4: Component Replacement If sensor resistance is out of specification or the element is contaminated, replace with an OEM Hitachi sensor. For used machines, always replace the connector pigtail simultaneously to ensure reliable connections. Clear codes using Hitachi diagnostic software (Dr.EX) and verify proper operation through a complete heat cycle.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Hitachi EX1200-5 service manual and consider professional assistance from certified Hitachi technicians for complex electrical diagnostics or if you're unfamiliar with high-voltage systems.