What is Kobelco SK350LC-8 Fault Code F021?

Kobelco SK350LC-8 Fault Code F021 indicates a malfunction in the engine coolant temperature sensor circuit. This fault code triggers when the Electronic Control Module (ECM) detects an abnormal voltage signal from the coolant temperature sensor, typically reading either too high (open circuit) or too low (short circuit) compared to normal operating parameters.

The coolant temperature sensor plays a critical role in the SK350LC-8's Hino engine management system. It provides essential data that the ECM uses to adjust fuel injection timing, throttle response, and engine warm-up cycles. When this sensor fails or sends erratic signals, the excavator's performance becomes compromised, potentially leading to engine damage if left unaddressed. For used excavators, this fault is particularly important to diagnose quickly, as prolonged operation with incorrect temperature readings can accelerate wear on critical engine components.

Common Symptoms

• Yellow or amber warning light illuminated on the instrument cluster, often accompanied by a distinct engine warning indicator
• Poor engine performance during cold starts, with rough idling or excessive black smoke from the exhaust
• Engine fan running continuously at high speed, even when the engine hasn't reached normal operating temperature
• Reduced hydraulic response or power loss, as the ECM may limit engine output to prevent perceived overheating
• Difficulty starting in cold weather conditions due to improper fuel mixture calculations

Potential Causes

The F021 code on used SK350LC-8 excavators typically stems from several wear-related issues. Sensor connector corrosion is extremely common, especially on machines that have operated in wet or coastal environments. The connector pins develop oxidation that disrupts the electrical signal.

Wiring harness damage frequently occurs where the sensor harness runs along the engine block near the turbocharger area—a known rub point on this model where heat and vibration cause insulation breakdown. Failed coolant temperature sensor itself ranks high on the list, particularly on machines with over 5,000 operating hours, as the sensor element degrades from constant heat cycling.

Less common but possible causes include ECM internal faults, damaged connector terminals from improper service procedures, or coolant contamination affecting sensor accuracy.

How to Troubleshoot and Fix Code F021

Step 1: Visual Inspection Begin by locating the coolant temperature sensor on the left side of the engine block (facing from operator's seat). Disconnect the two-pin connector and carefully inspect for corrosion, bent pins, or moisture intrusion. On used excavators, apply electrical contact cleaner and examine the rubber seal—deteriorated seals allow coolant seepage that corrodes connections.

Step 2: Resistance Testing Using a digital multimeter, measure the sensor's resistance. At room temperature (68°F/20°C), resistance should read approximately 2,000-3,000 ohms. Compare your reading against Kobelco's specifications in the service manual. If readings are infinite (open circuit) or near zero (short circuit), replace the sensor.

Step 3: Wiring Harness Verification Trace the harness from the sensor to the ECM connector (located behind the operator's cab). Check for voltage at the sensor connector with the key on, engine off—you should see approximately 5 volts reference voltage from the ECM. Inspect the harness near heat shields and sharp edges where chafing commonly occurs on higher-hour machines.

Step 4: ECM Signal Check With the sensor disconnected, measure voltage between the two harness pins. If voltage doesn't match specifications or fluctuates erratically, suspect wiring issues or ECM problems. For used excavators, verify all ground connections are clean and tight, as corroded grounds frequently cause intermittent sensor faults.

Disclaimer: While this guide provides comprehensive troubleshooting steps, complex electrical diagnostics may require Kobelco diagnostic software and factory training. Always consult a certified heavy equipment technician for issues beyond basic sensor replacement, especially when dealing with ECM-related faults.