Komatsu PC400-6 Fault Code E108: Complete Diagnostic Guide

What is Komatsu PC400-6 Fault Code E108?

Fault Code E108 on the Komatsu PC400-6 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 coolant temperature sensor sending irregular voltage signals outside the manufacturer's specified parameters to the ECM.

The coolant temperature sensor is critical for the PC400-6's performance because it directly influences fuel injection timing, engine protection protocols, and overall combustion efficiency. When this sensor fails or provides inaccurate data, the ECM cannot properly regulate engine temperature, potentially leading to overheating, reduced power output, or premature engine wear in used machines.

Common Symptoms

When Code E108 is active on your PC400-6, operators typically experience:

• Warning light illumination on the instrument cluster, specifically the engine diagnostic or coolant temperature indicator
• Engine derate mode activation, limiting maximum RPM and hydraulic performance to protect the engine
• Erratic temperature gauge readings that fluctuate abnormally or remain stuck at one position
• Difficulty starting when the engine is cold, as the ECM receives incorrect temperature data for fuel mixture calculations
• Black smoke from exhaust due to improper fuel-to-air ratios caused by faulty temperature readings

Potential Causes

The most common technical reasons for E108 appearing on used PC400-6 excavators include:

• Coolant temperature sensor failure due to age-related degradation or internal element breakdown
• Wiring harness damage at known rub points near the engine block where vibration causes insulation wear
• Corroded or loose connector pins at the sensor plug, especially common in machines exposed to harsh environments
• ECM internal circuit issues affecting the sensor input channel (less common but possible in high-hour machines)
• Damaged sensor ground circuit causing voltage reference problems
• Coolant contamination causing sensor element coating and inaccurate readings

How to Troubleshoot and Fix Code E108

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, or oil contamination. On used excavators, pay special attention to areas where the harness contacts metal surfaces or passes near moving components. Check the connector for corrosion, bent pins, or moisture intrusion.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance. At room temperature (20°C/68°F), resistance should typically read between 2,000-3,000 ohms (consult your service manual for exact specifications). Test the harness continuity from the sensor connector back to the ECM. Check for proper ground circuit integrity and verify supply voltage (typically 5V reference) at the connector with ignition on.

Step 3: Sensor and Circuit Verification If resistance values are out of specification or the sensor shows open/short circuit conditions, replace the coolant temperature sensor with a genuine Komatsu part. Before installation, clean the sensor mounting location and apply appropriate thread sealant. After replacement, clear the fault code using Komatsu diagnostic software (KOMTRAX or compatible scan tool) and monitor for code recurrence. For used machines, consider replacing the sensor connector pigtail if corrosion is present, as this prevents future intermittent faults.

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu PC400-6 service manual and consider professional assistance from certified technicians for complex electrical diagnostics or if you're uncertain about any procedure.