What is Komatsu Fault Code E608?

Komatsu Fault Code E608 indicates a malfunction in the engine coolant temperature sensor circuit, specifically detecting an abnormally high voltage signal that suggests an open circuit, disconnected sensor, or wiring fault. This diagnostic trouble code (DTC) is logged when the Engine Control Module (ECM) receives a voltage reading outside the expected range from the coolant temperature sensor, typically above 4.8-5.0 volts when normal operating range should be 0.5-4.5 volts.

This fault is critical because the ECM relies on accurate coolant temperature data to manage fuel injection timing, engine protection protocols, and emission controls. When E608 triggers, the ECM defaults to a substitute temperature value, which can lead to inefficient combustion, increased fuel consumption, and potential engine damage if overheating occurs undetected.

Common Symptoms

• Engine warning light or malfunction indicator lamp illuminated on the instrument panel
• Engine may enter derate mode (reduced power output) as a protective measure
• Difficult cold starting or rough idling, especially in cooler ambient temperatures
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Inaccurate temperature gauge readings, often showing maximum or minimum values

Potential Causes

The most common technical causes for E608 on used Komatsu excavators include:

• Disconnected or corroded wiring harness connector at the coolant temperature sensor (common failure point after years of vibration and thermal cycling)
• Open circuit in the sensor wiring harness, particularly at flex points near the engine block or behind the fuel injection pump where cable chafing occurs
• Failed coolant temperature sensor with internal open circuit (sensor element degradation after extended use)
• Damaged ECM wiring or poor ground connections affecting the 5-volt reference circuit
• Corroded ECM connector pins (especially in machines operated in coastal or high-humidity environments)

How to Troubleshoot and Fix Code E608

Step 1: Visual Inspection Begin by locating the coolant temperature sensor (typically mounted in the thermostat housing or cylinder head). Inspect the sensor connector for corrosion, moisture intrusion, bent pins, or physical damage. On used excavators, check for harness wear at contact points with engine components or frame members.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. A functioning sensor should show approximately 2,000-3,000 ohms at room temperature (68°F/20°C). An infinite resistance reading confirms sensor failure. Next, check the wiring harness for continuity between the sensor connector and ECM pins using the machine's wiring diagram.

Step 3: Voltage Circuit Check With the ignition on and sensor disconnected, measure voltage at the harness connector. You should see approximately 5 volts on the reference wire. If voltage is absent or incorrect, trace the reference voltage circuit back to the ECM, checking for damaged wires or poor grounds.

Step 4: Component Replacement Replace the coolant temperature sensor if resistance values are out of specification. For used machines, always apply dielectric grease to connector pins before reassembly to prevent future corrosion. Clear the fault code using Komatsu diagnostic software (KOMTRAX or compatible scan tool) and verify repair by monitoring live data during a complete heat cycle.

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Disclaimer: This guide provides general troubleshooting information. Always consult the specific service manual for your Komatsu model and consider professional diagnostic assistance for complex electrical issues or if you're unfamiliar with high-voltage systems.