What is Kobelco SK350-6E Fault Code F-3?

Kobelco SK350-6E Fault Code F-3 indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading detected by the Electronic Control Module (ECM). This diagnostic trouble code specifically monitors the coolant temperature sensor signal voltage and alerts operators when readings fall outside the acceptable range (typically 0.5-4.5V) or when the engine is overheating beyond safe operational limits.

This fault is critical for the SK350-6E because the ECM relies on accurate coolant temperature data to optimize fuel injection timing, control engine speed, and prevent catastrophic thermal damage to the Hino J05E engine. When F-3 triggers, the machine may enter a protective derate mode, significantly reducing performance to prevent engine damage. For used excavators, this code often appears due to age-related sensor degradation or wiring issues common in high-hour machines.

Common Symptoms

• Warning light illuminated on the instrument panel, often accompanied by audible alarm
• Engine derate or power loss, with RPM limited to 1500-1800 range to prevent overheating
• Erratic temperature gauge readings that fluctuate abnormally or remain pegged at maximum
• Cooling fan running continuously at high speed, even during cold starts
• Difficulty starting when engine is cold, or refusal to start when engine appears overheated

Potential Causes

The most frequent causes of F-3 in used SK350-6E excavators include:

• Coolant temperature sensor failure due to internal resistance drift (common after 8,000+ operating hours)
• Wiring harness damage near the engine block where vibration causes insulation wear and short circuits
• Corroded or loose connector pins at the sensor plug, especially on machines operated in marine or mining environments
• Low coolant level or air pockets in the cooling system causing genuine overheating conditions
• ECM internal fault (rare, but possible in machines with electrical system damage history)
• Damaged sensor mounting threads causing poor thermal contact with coolant passages

How to Troubleshoot and Fix Code F-3

Step 1: Visual Inspection Begin by checking coolant level in the overflow tank and radiator when cold. Inspect the coolant temperature sensor (located on the thermostat housing near the engine front) for physical damage, coolant leaks, or corrosion. On used excavators, carefully examine the wiring harness from the sensor to the ECM for abraded insulation, especially where it passes near the engine mount brackets—a known rub point on SK350-6E models.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure sensor resistance. At 20°C (68°F), resistance should read approximately 2,200-2,700 ohms; at 80°C (176°F), expect 280-320 ohms. Compare your readings to Kobelco specifications. Next, check circuit continuity from the sensor connector to the ECM pins (typically pin 23 for signal and pin 45 for ground) to identify open circuits.

Step 3: Signal Voltage Verification With the sensor connected and ignition on (engine off), backprobe the signal wire and measure voltage—it should read 4.5-5.0V. Start the engine and monitor voltage as it warms; voltage should gradually decrease to 0.8-1.2V at operating temperature. Erratic readings or voltage outside these ranges confirm sensor replacement is necessary.

Step 4: Connector Service and Parts Replacement For used machines, clean all connector terminals with electrical contact cleaner and apply dielectric grease before reconnection. Replace the sensor with genuine Kobelco parts (P/N YN52S00002P1) if testing confirms failure. After replacement, clear codes using INSITE diagnostic software or the Kobelco proprietary diagnostic tool and perform a test cycle.

Disclaimer: This guide provides general diagnostic procedures. Always consult the Kobelco SK350-6E service manual and consider professional assistance for complex electrical diagnostics. Improper repairs may cause additional damage or safety hazards.