What is Komatsu PC120-6 Fault Code H-5?

Fault Code H-5 on the Komatsu PC120-6 excavator indicates a hydraulic system overheat condition, specifically detecting that hydraulic oil temperature has exceeded the safe operating threshold (typically above 95-100°C or 203-212°F). This code is generated by the hydraulic oil temperature sensor located in the main hydraulic tank, which continuously monitors oil temperature and sends signals to the machine's ECM (Engine Control Module).

This fault is critical for the PC120-6 because excessive hydraulic temperatures can cause rapid degradation of seals, accelerated oil breakdown, and potential damage to hydraulic pumps and control valves. On used excavators, this code often signals underlying cooling system deterioration or blockages that have developed over thousands of operating hours.

Common Symptoms

• Warning light illuminated on the instrument panel (typically a red hydraulic overheat indicator)
• Reduced hydraulic performance including slower cycle times and weaker bucket/arm force
• Audible buzzer alarm when oil temperature exceeds critical threshold
• Automatic power derate limiting engine RPM to protect hydraulic components
• Visible steam or heat waves from the hydraulic tank area during operation

Potential Causes

On used PC120-6 excavators, H-5 codes commonly result from age-related cooling system failures. The hydraulic oil cooler fins often become clogged with debris, mud, and dust accumulation—especially problematic on machines operating in demolition or earthmoving environments. Cooling fan clutch failure prevents adequate airflow through the cooler core.

The hydraulic oil temperature sensor itself may provide false readings due to corrosion on connector terminals or internal sensor degradation. Degraded or incorrect viscosity hydraulic oil (often found in used machines with unknown service history) generates excessive heat through increased friction. Worn hydraulic pumps create additional heat from internal leakage and inefficiency.

How to Troubleshoot and Fix Code H-5

Step 1: Verify Actual Temperature Use an infrared thermometer to measure actual hydraulic tank oil temperature. Compare readings with the machine's gauge. If the sensor reads high but actual temperature is normal, suspect sensor or wiring issues. Check connector C-5 near the hydraulic tank for corrosion or loose pins.

Step 2: Inspect Cooling System Clean the hydraulic oil cooler thoroughly using compressed air or pressure washer from the inside-out direction. Inspect cooling fan operation—the fan clutch should engage when oil temperature rises. On used excavators, check for bent cooler fins or internal core blockage requiring replacement.

Step 3: Test Hydraulic Oil Condition Sample hydraulic oil and check for correct ISO VG 46 or VG 68 viscosity (depending on ambient conditions). Dark, burnt-smelling oil indicates overheating damage. Replace oil and filters if contaminated. Check suction strainer in the tank for blockage restricting flow.

Step 4: Sensor and Electrical Diagnostics Using a multimeter, test the temperature sensor resistance (typically 200-300 ohms at 20°C). Check wiring harness routing along the tank where vibration and rubbing commonly damage insulation on older machines.

Disclaimer: This guide provides general diagnostic information. Complex hydraulic issues should be evaluated by qualified technicians with Komatsu-specific diagnostic tools and training.