What is Komatsu PC120LC-6 Fault Code E308?

Fault Code E308 on the Komatsu PC120LC-6 indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal temperature reading detected by the Engine Control Module (ECM). This code typically triggers when the ECM receives voltage signals outside the expected range from the coolant temperature sensor, suggesting either sensor failure, wiring issues, or genuine overheating conditions.

The coolant temperature sensor plays a critical role in the PC120LC-6's engine management system. It provides real-time data to the ECM, which adjusts fuel injection timing, idle speed, and cooling fan operation accordingly. When this sensor fails or sends erratic signals, the engine may run inefficiently, experience reduced power output, or enter a protective derate mode to prevent thermal damage. For used excavators, this code is particularly important as aging sensors and corroded connectors are common failure points.

Common Symptoms

When Code E308 is active, operators typically experience:

• Engine warning lamp illuminated on the instrument panel, often accompanied by audible alerts
• Increased engine idle speed as the ECM defaults to a failsafe cooling strategy
• Reduced hydraulic performance or power derate during operation, especially under load
• Cooling fan running continuously at maximum speed, regardless of actual engine temperature
• Difficulty starting in cold conditions, as the ECM cannot properly enrich the fuel mixture

Potential Causes

The most common technical causes for E308 on used PC120LC-6 excavators include:

• Coolant temperature sensor degradation due to prolonged exposure to heat cycles and corrosive coolant additives
• Damaged wiring harness at known rub points near the engine block or valve cover, where vibration causes insulation wear
• Corroded or loose connector pins at the sensor plug, particularly common in machines operating in wet or coastal environments
• ECM internal fault or corrupted calibration data (less common but possible on high-hour machines)
• Actual engine overheating caused by low coolant levels, failed thermostats, or clogged radiators triggering legitimate high-temperature readings

How to Troubleshoot and Fix Code E308

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the engine block near the thermostat housing. Check for physical damage, coolant leaks, or corrosion on the connector. On used excavators, examine the wiring harness for wear points where it contacts metal surfaces or passes through brackets.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance at various temperatures (consult the Komatsu service manual for specific values—typically 2-3 kΩ at 20°C). Test the harness side for continuity to the ECM and check for short circuits to ground. Verify supply voltage from the ECM (usually 5V reference).

Step 3: Monitor Live Data Connect Komatsu diagnostic software (KOMTRAX or compatible scan tool) to monitor real-time coolant temperature readings. Compare sensor output against actual engine temperature using an infrared thermometer. Erratic fluctuations or stuck readings confirm sensor failure.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Komatsu part (P/N 6732-81-3140 or equivalent). For used machines, always replace the connector pigtail simultaneously if corrosion is present. Clear codes and perform a heat cycle test to verify repair.

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Disclaimer: This guide provides general diagnostic procedures. Always consult the official Komatsu PC120LC-6 service manual and consider professional technician evaluation for complex electrical issues or if unfamiliar with high-voltage systems.