Komatsu PC120LC-6 Fault Code H-8: Technical Guide

What is Komatsu PC120LC-6 Fault Code H-8?

Fault Code H-8 indicates a malfunction in the hydraulic oil temperature sensor circuit or abnormally high hydraulic oil temperature. This code is triggered when the Electronic Control Module (ECM) detects either a sensor voltage reading outside the normal range (0.5-4.5V) or hydraulic oil temperatures exceeding 110°C (230°F).

This fault is critical for the PC120LC-6 because excessive hydraulic temperatures can cause premature seal degradation, reduced hydraulic efficiency, and potential damage to the main hydraulic pump and control valves. The machine's control system may automatically reduce engine RPM or limit hydraulic functions to prevent catastrophic failures.

Common Symptoms

• Hydraulic oil temperature warning light illuminated on the instrument panel
• Noticeable reduction in hydraulic response speed and digging force
• Engine RPM limitation activated, preventing operation above 1,500-1,800 RPM
• Unusual whining or cavitation noises from the hydraulic pump
• Abnormally hot hydraulic reservoir tank to the touch (above 90°C/194°F)

Potential Causes

• Hydraulic oil temperature sensor failure (common on machines with 5,000+ hours due to heat cycling)
• Damaged or corroded wiring harness between sensor and ECM, particularly at the swing joint area where cables flex repeatedly
• Insufficient hydraulic oil level or use of incorrect viscosity oil
• Blocked or debris-filled hydraulic oil cooler fins reducing heat dissipation
• Failing cooling fan motor or damaged fan blades
• Relief valve malfunction causing excessive system pressure and heat generation
• ECM internal fault (rare, but occurs in machines exposed to moisture intrusion)

How to Troubleshoot and Fix Code H-8

Step 1: Visual Inspection Check hydraulic oil level in the reservoir—should be between MIN and MAX marks when cool. Inspect the hydraulic oil cooler for debris, bent fins, or blockages. On used excavators, examine wiring harness routing near the swing bearing and boom cylinder areas for chafing, exposed copper, or corroded connectors.

Step 2: Sensor Circuit Testing Using a digital multimeter, measure resistance across the temperature sensor terminals (sensor disconnected). At 20°C, resistance should read approximately 2.4-2.8 kΩ. Check harness continuity from sensor connector to ECM pin 47 (typically)—should show less than 5Ω resistance. Verify no short-to-ground (infinite resistance to chassis).

Step 3: Hydraulic System Evaluation Start the machine and monitor actual hydraulic oil temperature with an infrared thermometer. If physical temperature reads normal (60-80°C) but code persists, replace the temperature sensor (Komatsu part 7861-93-3320). If temperature genuinely exceeds 100°C, inspect the cooling fan operation, clean the oil cooler, and verify relief valve settings (typically 34.3 MPa/4,975 PSI).

Critical for Used Machines: Inspect all electrical connectors for corrosion—apply dielectric grease after cleaning. Check for previous repair attempts, aftermarket sensors, or modified wiring that may cause voltage discrepancies.

Disclaimer: This guide provides general technical information. Always consult the official Komatsu service manual and consider professional diagnostic assistance for complex electrical or hydraulic issues.