What is Kobelco SK140 Fault Code 06551?

Fault Code 06551 on the Kobelco SK140 excavator indicates a malfunction in the engine coolant temperature sensor circuit, specifically a voltage signal that is out of the normal operating range. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormal voltage reading from the coolant temperature sensor, which monitors the engine's operating temperature and helps the control system optimize fuel injection, idle speed, and emissions control.

This code is critical for the SK140's diesel engine because incorrect temperature readings can cause the ECM to make improper fuel delivery adjustments, leading to poor engine performance, increased fuel consumption, and potential overheating conditions. In used excavators, this sensor circuit is particularly vulnerable to degradation from years of exposure to heat cycling and environmental contaminants.

Common Symptoms

When Code 06551 is active on your Kobelco SK140, you may experience:

• Check engine warning light illuminated on the instrument panel
• Hard starting or extended cranking time, especially during cold starts
• Rough idling or inconsistent engine RPM at low speeds
• Black smoke from the exhaust due to improper fuel-to-air ratio
• Engine derate mode where the machine limits power output to prevent potential damage

Potential Causes

The most common technical reasons for Code 06551 appearing on used SK140 excavators include:

• Failed coolant temperature sensor with internal resistance drift or complete circuit failure
• Damaged wiring harness along the routing path from the sensor to the ECM, particularly where the harness passes near hot engine components or sharp metal edges
• Corroded or loose electrical connectors at the sensor or ECM terminals due to moisture intrusion
• Broken or frayed sensor ground wire causing intermittent voltage spikes
• ECM internal fault (less common) affecting the temperature sensor input circuit

How to Troubleshoot and Fix Code 06551

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, abrasion points where the wire contacts metal surfaces, or evidence of heat damage. Check the electrical connector at the sensor for corrosion, bent pins, or moisture—common issues in used excavators that have experienced years of outdoor operation.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance at various temperatures. A functional sensor typically shows approximately 2,000-3,000 ohms at room temperature, decreasing as temperature increases. Compare your readings to Kobelco's specifications. Next, check the signal wire voltage with the ignition on and sensor disconnected—you should see approximately 5 volts reference voltage from the ECM.

Step 3: Harness Continuity Check Test continuity between the sensor connector and the ECM connector for both the signal wire and ground circuit. Look for resistance above 5 ohms, which indicates corrosion or damaged conductors. On used machines, pay special attention to harness sections that experience vibration or contact with moving components.

Step 4: Component Replacement If testing confirms a faulty sensor, replace it with an OEM Kobelco sensor or quality equivalent. When installing, apply dielectric grease to the connector to prevent future corrosion. If the sensor tests good but the code persists, the ECM may require professional diagnosis with Kobelco diagnostic software to check for internal faults.

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Disclaimer: This guide provides general diagnostic information for experienced technicians. Always consult the official Kobelco service manual for your specific machine serial number and consider professional assistance for complex electrical diagnostics or ECM programming procedures.