Komatsu PC400LC-6 Fault Code E108: Complete Diagnostic Guide

What is Komatsu PC400LC-6 Fault Code E108?

Fault Code E108 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 Komatsu SAA6D125E-3 engine from overheating damage.

The coolant temperature sensor sends voltage signals to the ECM, which uses this data to regulate fuel injection timing, idle speed, and activate warning systems. On the PC400LC-6, this sensor is critical because improper temperature readings can cause the engine to run in derate mode (reduced power output) or trigger automatic shutdown protocols to prevent catastrophic engine damage. Given that this is a hydraulic excavator, maintaining optimal engine temperature is essential for both hydraulic oil viscosity and overall system performance.

Common Symptoms

When Code E108 is active, operators typically experience:

• Coolant temperature warning light illuminated on the instrument cluster (red or amber)
• Engine derate mode activated, resulting in significantly reduced hydraulic power and slower cycle times
• Erratic temperature gauge readings or gauge stuck at minimum/maximum positions
• Difficulty starting when the engine is cold, or rough idle conditions
• Automatic engine shutdown in severe cases where the ECM detects dangerously high temperature readings

Potential Causes

The most common technical causes for E108 on used PC400LC-6 excavators include:

• Coolant temperature sensor failure due to age-related degradation or internal short/open circuit
• Damaged wiring harness near the sensor connector, particularly where the harness routes along the engine block (known rub point on this model)
• Corroded or loose electrical connectors at the sensor or ECM terminals, common in machines with high operating hours
• ECM internal fault affecting the temperature monitoring circuit (less common)
• Actual engine overheating causing legitimate high-temperature readings (verify coolant level, radiator condition, and water pump operation)
• Ground circuit issues affecting sensor signal return path

How to Troubleshoot and Fix Code E108

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the engine block (typically near the thermostat housing on the SAA6D125E-3 engine). Check for physical damage, coolant leaks around the sensor, and examine the wiring harness for abrasion, cuts, or heat damage. On used excavators, pay special attention to areas where the harness contacts the engine block or frame—this is a common failure point.

Step 2: Connector and Ground Testing Disconnect the sensor connector and inspect for corrosion, bent pins, or moisture intrusion. Clean contacts with electrical contact cleaner if necessary. Using a digital multimeter, verify the ground circuit has less than 0.5 ohms resistance to chassis ground. Check the supply voltage from the ECM (should be approximately 5 volts with ignition on).

Step 3: Sensor Resistance Testing With the sensor disconnected, measure resistance across the sensor terminals. A functional sensor typically shows 2,000-3,000 ohms at room temperature (refer to Komatsu service manual for exact specifications). Compare readings at different temperatures if possible. Replace the sensor if readings are out of specification or show infinite/zero resistance.

Step 4: Wiring Continuity Check Test continuity between the sensor connector and ECM pins (consult wiring diagrams for specific pin locations). Look for opens or shorts to ground. For used machines with 8,000+ hours, consider replacing the entire harness section if multiple wire issues are found.

Step 5: Clear Code and Monitor After repairs, use Komatsu diagnostic software (KOMTRAX or compatible scan tool) to clear the fault code. Monitor live data during a test run, verifying temperature readings match actual engine conditions. If the code returns immediately, suspect ECM issues requiring dealer-level diagnostics.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number. If you're uncomfortable performing electrical diagnostics, contact a certified Komatsu technician to prevent further damage to expensive electronic components.