What is Komatsu PC120-6 Fault Code H-27?

Komatsu PC120-6 fault code H-27 indicates a hydraulic oil temperature sensor circuit malfunction or an abnormally high hydraulic oil temperature condition. This code is triggered when the Electronic Control Module (ECM) detects either an open/short circuit in the hydraulic oil temperature sensor wiring or when actual hydraulic oil temperature exceeds the safe operating threshold (typically above 95-100°C/203-212°F).

This fault is critical for the PC120-6 because excessive hydraulic oil temperature degrades viscosity, reduces lubrication efficiency, and accelerates seal deterioration. The hydraulic system is the lifeblood of excavator operation, and temperature monitoring prevents catastrophic pump failure and cylinder seal damage that can cost thousands in repairs.

Common Symptoms

• Warning lamp illumination on the instrument panel, often accompanied by an audible alarm
• Reduced hydraulic performance, including slower boom/arm/bucket movements and decreased digging force
• Visible steam or heat distortion near the hydraulic oil tank or cooler assembly
• Auto-derate mode where engine RPM is limited to protect the hydraulic system
• Difficulty performing high-demand tasks like heavy digging or rapid cycle operations

Potential Causes

For used PC120-6 excavators, this code typically stems from age-related component degradation:

• Failed hydraulic oil temperature sensor (common after 5,000+ operating hours due to thermal cycling stress)
• Damaged wiring harness at the sensor connector, particularly where the harness routes near the swing motor or along the upper frame—known rub points for this model
• Clogged hydraulic oil cooler fins blocked by debris, preventing adequate heat dissipation
• Low hydraulic oil level or using incorrect viscosity oil for ambient conditions
• Faulty hydraulic cooling fan or worn fan drive belt causing insufficient airflow
• Corroded sensor connector pins exposing circuits to moisture intrusion (extremely common in outdoor storage)

How to Troubleshoot and Fix Code H-27

Step 1: Visual Inspection Check hydraulic oil level at the tank sight glass—ensure level is within the HOT range with engine running. Inspect the hydraulic oil cooler for external debris, bent fins, or blockages. Examine the cooling fan operation and belt tension.

Step 2: Sensor Circuit Testing Locate the hydraulic oil temperature sensor (typically mounted on the hydraulic tank side). Disconnect the sensor connector and inspect for corrosion, bent pins, or oil contamination. Using a digital multimeter, measure sensor resistance—specification is approximately 200-300 ohms at 20°C (68°F). Compare readings against Komatsu's temperature-resistance chart.

Step 3: Wiring Harness Verification Trace the sensor harness from the tank to the ECM, checking for pinching, chafing, or damage at frame crossmembers and swing bearing areas. Perform a continuity test on both signal and ground wires. Check for short-to-ground conditions (should read infinite resistance).

Step 4: Actual Temperature Verification Use an infrared thermometer to measure actual hydraulic oil tank temperature. If temperature is genuinely elevated (above 85°C/185°F), address cooling system issues before replacing sensors.

Step 5: Component Replacement For used excavators, replace the sensor even if electrical tests pass marginally—thermal drift is common. Use genuine Komatsu parts (part number varies by serial number prefix). Apply dielectric grease to new connector to prevent future corrosion.

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Disclaimer: This guide provides general diagnostic information. Always consult Komatsu service manuals specific to your machine's serial number and seek qualified technician assistance for complex repairs. Improper diagnostics may cause additional damage.