What is Komatsu PC138US-2 Fault Code E308?

Fault Code E308 on the Komatsu PC138US-2 indicates a malfunction in the engine coolant temperature sensor circuit. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormal voltage signal from the coolant temperature sensor, typically indicating an open circuit, short circuit, or sensor failure.

The coolant temperature sensor plays a critical role in the PC138US-2's engine management system. It monitors engine operating temperature and sends voltage signals to the ECM, which then adjusts fuel injection timing, idle speed, and controls the cooling fan operation. When this sensor circuit fails, the engine management system loses vital temperature data, potentially causing the engine to operate inefficiently or enter a protective derate mode to prevent overheating damage.

Common Symptoms

When Code E308 is active on your Komatsu PC138US-2, you may experience:

• Warning light illuminated on the instrument cluster, often accompanied by an audible alarm
• Engine running at higher-than-normal RPM during idle due to ECM defaulting to cold-start parameters
• Cooling fan running continuously or cycling improperly regardless of actual engine temperature
• Reduced engine power or performance derate as the ECM enters protective mode
• Difficulty starting when the engine is hot, as the ECM cannot accurately determine temperature conditions

Potential Causes

The most common technical causes for E308 on used PC138US-2 excavators include:

• Faulty coolant temperature sensor – internal sensor element degradation is common after 5,000+ operating hours
• Damaged wiring harness between the sensor and ECM, particularly at known rub points near the engine block mounting brackets
• Corroded or loose electrical connectors at the sensor plug or ECM terminals (extremely common in machines exposed to moisture)
• Open or short circuit in the sensor signal wire, often caused by harness chafing against metal surfaces
• ECM internal failure (rare, but possible in high-hour machines with electrical system history)

How to Troubleshoot and Fix Code E308

Step 1: Visual Inspection Begin by locating the coolant temperature sensor (typically mounted on the engine coolant passage near the thermostat housing). Inspect the sensor connector for corrosion, moisture intrusion, or bent pins. On used excavators, check the entire wiring harness route for signs of rubbing, cuts, or heat damage, paying special attention to areas where harnesses contact metal brackets or the engine block.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to check sensor resistance. At room temperature (68°F/20°C), 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 using the multimeter's continuity function. Check for voltage supply at the sensor connector with the key on, engine off—you should see approximately 5 volts reference voltage from the ECM.

Step 3: Diagnosis with Komatsu Software Connect Komatsu diagnostic software (KOMTRAX or dealer-level diagnostics) to monitor live coolant temperature readings. Compare the sensor reading against actual engine temperature using an infrared thermometer. A discrepancy of more than 20°F indicates sensor failure.

Step 4: Component Replacement If testing confirms sensor failure, replace the coolant temperature sensor with a genuine Komatsu part or OEM-equivalent. Before installing, apply dielectric grease to connector terminals to prevent future corrosion—critical for used machines. If harness damage is found, repair using heat-shrink solder connectors and route away from sharp edges or heat sources. Clear the fault code using diagnostic software and perform a complete heat cycle test.

For used excavators, never assume a new sensor will solve the problem without first testing the electrical circuit—harness issues account for approximately 40% of E308 codes on higher-hour machines.

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Disclaimer: This guide provides general troubleshooting information. Always consult your Komatsu PC138US-2 service manual and consider professional diagnostic assistance for complex electrical issues. Improper repairs may cause additional damage or safety hazards.