What is Komatsu PC120LC-6 Fault Code H-5?

Fault Code H-5 on the Komatsu PC120LC-6 indicates a hydraulic oil temperature sensor malfunction or abnormally high hydraulic oil temperature. This code specifically monitors the hydraulic system's thermal management, which is critical for maintaining proper viscosity and protecting hydraulic components from thermal breakdown.

The hydraulic oil temperature sensor sends real-time temperature data to the machine's Electronic Control Module (ECM). When temperatures exceed safe operating limits (typically above 95-100°C/203-212°F) or the sensor circuit shows electrical irregularities, the H-5 code triggers. For the PC120LC-6, this is particularly critical because prolonged high temperatures accelerate seal degradation, reduce pump efficiency, and can cause catastrophic hydraulic system failure.

Common Symptoms

• Warning light illuminated on the instrument panel, often accompanied by an audible alarm
• Reduced hydraulic response or sluggish implement movement during operation
• Visible smoke or burning smell emanating from the hydraulic tank area
• Automatic derate mode where machine limits operational speed to prevent damage
• Physical observation of overheating hydraulic oil through sight glass or temperature gauge readings exceeding normal range (60-80°C)

Potential Causes

Sensor-Related Issues:

• Failed or degraded hydraulic oil temperature sensor (common in machines with 5,000+ hours)
• Corroded or loose sensor wiring connectors at the hydraulic tank
• Damaged sensor harness due to rubbing against tank brackets or frame components

Actual Temperature Problems:

• Clogged or restricted hydraulic oil cooler fins reducing heat dissipation
• Low hydraulic oil level causing insufficient cooling capacity
• Malfunctioning cooling fan or fan drive motor
• Contaminated hydraulic oil with degraded thermal properties
• Blocked suction strainer increasing system resistance and heat generation

How to Troubleshoot and Fix Code H-5

Step 1: Initial Inspection Check actual hydraulic oil temperature using an infrared thermometer at the tank. If temperature reads normal (below 80°C) but code persists, suspect sensor/electrical fault. Inspect hydraulic oil level through sight glass and examine cooler fins for debris blockage.

Step 2: Electrical Diagnosis Using a digital multimeter, measure sensor resistance. The temperature sensor typically shows 200-300 ohms at 20°C (resistance decreases as temperature rises). Check wiring harness for continuity between sensor connector and ECM. For used excavators, carefully inspect connectors for corrosion and harness routing for wear points against metal edges.

Step 3: Component Testing Disconnect the sensor and measure supply voltage from the ECM (should be approximately 5V reference). If voltage is correct, replace the temperature sensor. If readings are abnormal, inspect ECM grounds and related circuits.

Step 4: Cooling System Service If actual temperatures are elevated, clean the hydraulic cooler using compressed air or pressure washer. Replace hydraulic oil and filters if contaminated. Test cooling fan operation and verify proper fan clutch engagement.

Step 5: System Verification Clear fault codes using Komatsu diagnostic software or appropriate scan tool. Operate machine under load while monitoring temperatures to confirm repair effectiveness.

Disclaimer: This guide provides general diagnostic information. Complex electrical issues or internal hydraulic problems should be evaluated by certified Komatsu technicians with proper diagnostic equipment.