What is Sumitomo SH80-6B Fault Code P0668-4?

Fault Code P0668-4 on the Sumitomo SH80-6B excavator indicates an Engine Coolant Temperature (ECT) circuit malfunction with intermittent signal dropout or abnormal voltage readings. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects irregular resistance or voltage fluctuations from the coolant temperature sensor circuit that fall outside the manufacturer's specified parameters.

This code is critical for the SH80-6B's performance because the ECM relies on accurate coolant temperature data to manage fuel injection timing, idle speed control, and engine protection protocols. When this sensor circuit malfunctions, the engine may operate with incorrect fuel mapping, potentially causing poor combustion, increased emissions, and premature component wear.

Common Symptoms

When Code P0668-4 is active on your Sumitomo SH80-6B, operators typically experience:

• Check Engine Light or malfunction indicator illuminated on the instrument cluster
• Hard starting conditions, especially when the engine is cold or has been sitting overnight
• Erratic idle speed or rough running during warm-up periods
• Reduced engine power or derate mode activation to prevent overheating damage
• Cooling fan running continuously at high speed regardless of actual engine temperature

Potential Causes

The most common technical causes for P0668-4 on used Sumitomo excavators include:

• Corroded or loose connector terminals at the coolant temperature sensor (common on machines with 3,000+ operating hours)
• Damaged wiring harness near the thermostat housing where vibration and heat cause insulation breakdown
• Failed coolant temperature sensor due to internal resistance drift or element contamination
• ECM internal circuit failure (rare, but possible on older machines)
• Coolant contamination affecting sensor accuracy, particularly glycol degradation in neglected systems
• Aftermarket sensor installation with incorrect resistance specifications for Sumitomo systems

How to Troubleshoot and Fix Code P0668-4

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor and its wiring harness near the thermostat housing. On used excavators, check specifically for harness chafing against brackets, corroded connector pins (white or green deposits), and coolant leaks that may have contaminated connections. Clean all terminals with electrical contact cleaner.

Step 2: Resistance Testing Disconnect the sensor and measure resistance across its terminals using a digital multimeter. At 68°F (20°C), resistance should read approximately 2,200-2,800 ohms (consult your service manual for exact specifications). Compare readings at different temperatures to verify the sensor's thermistor curve matches factory specifications.

Step 3: Circuit Voltage Check With the sensor disconnected and ignition on (engine off), check for 5-volt reference voltage at the harness connector from the ECM. Measure ground circuit continuity to ensure less than 0.5 ohms resistance. Voltage outside 4.5-5.2V indicates ECM supply issues or harness damage.

Step 4: Harness Continuity Test Perform a wiggle test while monitoring sensor voltage with the engine running. Movement-induced voltage spikes confirm intermittent wiring faults. Trace the harness completely, as the SH80-6B commonly develops wire breaks near the engine mount points due to vibration fatigue.

Step 5: Component Replacement Replace the sensor with a genuine Sumitomo or OEM-equivalent part matching original resistance specifications. For used machines, simultaneously replace the connector pigtail if corrosion is present. Clear codes using diagnostic software and perform a test cycle monitoring live data.

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Disclaimer: This guide provides general troubleshooting information. Always consult the Sumitomo SH80-6B service manual for exact specifications and safety procedures. If unfamiliar with electrical diagnostics, seek assistance from a qualified heavy equipment technician to prevent damage to electronic components.