What is Sumitomo Excavator Fault Code 31?

Fault Code 31 on Sumitomo excavators indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal temperature reading outside the expected operating range. This diagnostic trouble code (DTC) is generated when the Electronic Control Module (ECM) detects either an open circuit, short circuit, or implausible voltage signal from the coolant temperature sensor (CTS).

The coolant temperature sensor plays a critical role in engine management by providing real-time data that influences fuel injection timing, idle speed control, and overall engine protection strategies. When this sensor fails or its circuit is compromised, the ECM cannot accurately monitor engine temperature, potentially leading to overheating conditions, poor fuel economy, or inadequate engine warm-up performance. For used Sumitomo excavators, this code is particularly important as aging wiring harnesses and sensor degradation are common issues that can trigger false readings or intermittent faults.

Common Symptoms

When Fault Code 31 is active on your Sumitomo excavator, you may experience:

• Warning light illumination on the instrument panel, often accompanied by an audible alarm
• Hard starting or rough idle, especially during cold engine conditions
• Black smoke from exhaust due to improper fuel mixture calculations
• Engine running cooler or hotter than normal operating temperature (typically 180-200°F)
• Poor fuel efficiency and reduced hydraulic system responsiveness during operation

Potential Causes

Several technical factors can trigger Code 31 on used Sumitomo excavators:

• Failed coolant temperature sensor due to internal element degradation or contamination from coolant additives
• Damaged or corroded wiring harness at connector points, particularly where the harness routes near the engine block or experiences vibration
• Broken or frayed sensor wiring caused by rubbing against engine components or frame members over years of operation
• Poor ground connections at the sensor or ECM, common in machines with corrosion buildup
• Contaminated coolant causing sensor element fouling or inaccurate readings
• ECM internal fault (rare, but possible in high-hour machines with electrical system stress)

How to Troubleshoot and Fix Code 31

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor location (typically on the engine block or thermostat housing). Check for physical damage, coolant leaks, or corrosion on the sensor connector. On used excavators, pay special attention to harness chafing points where wires contact metal surfaces or moving components.

Step 2: Electrical Testing Using a digital multimeter (DMM), disconnect the sensor and measure its resistance at ambient temperature. Most Sumitomo CTS sensors should read between 2,000-3,000 ohms at 68°F (consult your service manual for exact specifications). Test the wiring harness continuity from sensor connector to ECM, checking for opens or shorts to ground.

Step 3: Voltage Signal Verification With the sensor connected and ignition on (engine off), backprobe the sensor connector and verify the reference voltage from the ECM (typically 5 volts). Start the engine and monitor voltage changes as temperature rises—voltage should decrease smoothly as resistance drops with heat.

Step 4: Component Replacement If testing confirms sensor failure, replace with an OEM or quality aftermarket sensor and apply dielectric grease to connector terminals. For used machines, consider replacing the sensor pigtail harness if wiring shows any signs of heat damage or insulation cracking.

Disclaimer: This guide provides general troubleshooting information. Always consult your Sumitomo service manual for model-specific procedures and specifications. If you're unsure about electrical diagnostics or repairs, seek assistance from a qualified heavy equipment technician to prevent further damage to your excavator's control systems.