What is Komatsu PC100-6 Fault Code H-31?

Komatsu PC100-6 fault code H-31 indicates a hydraulic oil temperature sensor circuit malfunction or abnormally high hydraulic oil temperature detected by the machine's monitoring system. This code triggers when the hydraulic oil temperature exceeds the predetermined safety threshold (typically above 95-100°C) or when the temperature sensor circuit experiences an electrical fault, such as open or short circuits.

This fault is critical for the PC100-6 because excessive hydraulic oil temperature directly impacts system efficiency, accelerates seal degradation, and can cause catastrophic pump or motor failures. The hydraulic oil temperature sensor monitors fluid conditions continuously, sending voltage signals to the ECM (Engine Control Module) to prevent thermal damage to expensive hydraulic components. In used excavators, this code frequently appears due to sensor deterioration or actual cooling system deficiencies developed over years of operation.

Common Symptoms

• Warning light illuminated on the instrument panel, often accompanied by an audible alarm
• Noticeable reduction in hydraulic system responsiveness or sluggish cylinder/motor movements
• Automatic engine derate or power limitation to protect hydraulic components
• Visible steam or unusual odor from the hydraulic oil tank area
• Intermittent code appearance during high-load operations or hot ambient conditions

Potential Causes

• Hydraulic oil temperature sensor failure due to internal element degradation (common in machines with 5,000+ hours)
• Damaged or corroded wiring harness connections, particularly at the sensor connector near the hydraulic tank where moisture accumulation occurs
• Restricted or clogged hydraulic oil cooler fins reducing heat dissipation efficiency
• Low hydraulic oil level causing inadequate cooling circulation
• Failed cooling fan or thermostat preventing proper oil temperature regulation
• Actual hydraulic system issues: excessive internal leakage in pumps, worn relief valves, or restricted return filters generating heat

How to Troubleshoot and Fix Code H-31

Step 1: Initial Visual Inspection Check hydraulic oil level using the sight glass. Inspect the oil cooler for debris blockage, bent fins, or external damage. On used excavators, cooler efficiency often drops 30-40% due to accumulated dirt. Clean thoroughly with compressed air from the inside-out.

Step 2: Sensor Circuit Testing Locate the hydraulic oil temperature sensor (typically mounted on the hydraulic tank or return line). Disconnect the sensor connector and inspect for corrosion—a prevalent issue in PC100-6 models. Using a multimeter, measure resistance across sensor terminals. At 25°C, resistance should read approximately 2.0-3.0 kΩ (consult service manual for exact specifications). Test harness continuity from sensor to ECM, checking for shorts to ground.

Step 3: Live Temperature Verification If sensor resistance is within specification, use an infrared thermometer to measure actual hydraulic oil temperature while monitoring sensor voltage output (should be 0.5-4.5V depending on temperature). Compare readings—discrepancies indicate sensor failure.

Step 4: Component Replacement Replace the temperature sensor if out-of-spec or corroded connectors. For used machines, always replace connector pigtails if corrosion is present, as intermittent connections cause repeat failures. Verify cooling fan operation and thermostat function.

Disclaimer: While these procedures assist with diagnosis, consult qualified technicians or authorized Komatsu service centers for complex electrical diagnostics and hydraulic system repairs.