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

Fault Code H-6 on the Komatsu PC120LC-6 indicates a hydraulic oil temperature sensor malfunction or abnormally high hydraulic oil temperature. This code is triggered when the Electronic Control Module (ECM) detects either an out-of-range signal from the hydraulic oil temperature sensor or when oil temperature exceeds the safe operating threshold (typically above 110°C/230°F).

This diagnostic code is critical for the PC120LC-6 because excessive hydraulic oil temperature directly compromises seal integrity, accelerates fluid degradation, and reduces hydraulic pump efficiency. The hydraulic system operates within tight tolerances, and temperature-related viscosity changes can cause catastrophic component failure if ignored.

Common Symptoms

When Code H-6 is active, operators typically experience:

• Warning buzzer activation with simultaneous dashboard warning light illumination
• Reduced hydraulic response and slower cycle times as the system enters derate mode
• Abnormally hot hydraulic oil tank surfaces to touch
• Visible smoke or heat shimmer rising from the hydraulic reservoir
• Hydraulic oil appearing darker or smelling burnt during visual inspection

Potential Causes

The most common technical causes for H-6 on used PC120LC-6 excavators include:

• Hydraulic oil temperature sensor failure due to internal circuit degradation or contamination exposure
• Corroded or damaged sensor wiring harness connections, particularly at the sensor connector or ECM terminals
• Actual hydraulic system overheating from clogged hydraulic oil cooler fins or failed cooling fan operation
• Low hydraulic oil levels causing insufficient heat dissipation capacity
• Worn hydraulic pump generating excessive heat from internal leakage
• Blocked return line filters creating back pressure and heat buildup
• Faulty ECM misinterpreting sensor signals due to voltage supply issues

How to Troubleshoot and Fix Code H-6

Step 1: Visual Inspection Check actual hydraulic oil temperature with an infrared thermometer. If temperature is normal (below 80°C/176°F), the issue is sensor-related. On used excavators, inspect the sensor wiring harness routing near the hydraulic tank for rubbing points, especially where it contacts frame members or hydraulic lines.

Step 2: Sensor Circuit Testing Using a digital multimeter, measure sensor resistance. The PC120LC-6 temperature sensor should read approximately 2.45kΩ at 20°C. Disconnect the sensor and check for voltage supply (typically 5V reference) from the ECM. Inspect connector pins for corrosion—common on older machines exposed to moisture.

Step 3: System Performance Verification If sensor tests normal, verify hydraulic cooler cleanliness and cooling fan operation. Check hydraulic oil condition and level. Replace degraded oil and clean cooler cores thoroughly. For persistent actual overheating, conduct hydraulic pump flow and pressure tests to identify internal wear.

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Disclaimer: This guide provides general troubleshooting information. Always consult Komatsu service manuals and qualified technicians for proper diagnosis and repair, especially when working with high-pressure hydraulic systems.