Komatsu PC650LC-8R Fault Code H-31: Complete Diagnostic Guide

What is Komatsu PC650LC-8R Fault Code H-31?

Fault Code H-31 indicates a hydraulic oil temperature sensor circuit malfunction in the Komatsu PC650LC-8R excavator. This code triggers when the Electronic Control Module (ECM) detects an abnormal voltage signal from the hydraulic oil temperature sensor, which monitors the operating temperature of the hydraulic system fluid.

The hydraulic oil temperature sensor plays a critical role in protecting the PC650LC-8R's hydraulic system from overheating damage. When functioning properly, it allows the ECM to adjust pump output and activate warning systems before temperatures reach damaging levels. For used excavators operating in demanding conditions, this sensor becomes essential for preventing catastrophic hydraulic component failure and maintaining optimal system efficiency.

Common Symptoms

When Code H-31 is active, operators typically experience:

• Hydraulic oil temperature warning light illuminated on the monitor panel, even when oil feels cool to touch
• Erratic temperature readings on the display, showing unrealistic values (extremely high or low)
• Reduced hydraulic performance as the ECM may enter a protective derate mode
• Intermittent warning alarms that appear and disappear without corresponding temperature changes
• Machine may enter limp mode in severe cases, limiting operational capabilities to protect components

Potential Causes

The most common technical reasons for H-31 on used PC650LC-8R excavators include:

• Damaged or corroded sensor connector at the hydraulic tank mounting location (common failure point due to heat exposure)
• Failed hydraulic oil temperature sensor with internal circuit degradation from thermal cycling
• Wiring harness chafing along the routing path from tank to cab, particularly near frame pivot points
• Corroded or loose ground connections affecting sensor circuit integrity
• ECM internal fault (rare, but possible in high-hour machines)
• Contaminated sensor tip with hydraulic varnish or debris affecting resistance values

How to Troubleshoot and Fix Code H-31

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor mounted on the hydraulic tank. Inspect the sensor connector for corrosion, oil contamination, bent pins, or moisture intrusion. Check the wiring harness along its entire route for signs of rubbing, cuts, or heat damage—particularly critical on used excavators with 5,000+ hours.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance at ambient temperature (typically 2,000-3,000 ohms at 20°C for Komatsu sensors). Compare readings against factory specifications in the service manual. Check circuit voltage at the harness connector with ignition on—should read approximately 5 volts reference voltage from the ECM.

Step 3: Circuit Continuity Check Test wiring continuity from the sensor connector back to the ECM pins. Check for shorts to ground or power. Inspect all splice points and connector terminals for green corrosion—a common issue in older machines exposed to moisture.

Step 4: Component Replacement If sensor resistance is out of specification or wiring shows damage, replace the faulty component. For used excavators, always apply dielectric grease to connectors during reassembly to prevent future corrosion. Clear codes using Komatsu diagnostic software (KOMTRAX) or compatible scan tool and verify repair with a test cycle.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number and seek professional assistance from certified technicians for complex repairs.