Komatsu PC850SE-8R Fault Code H-8: Complete Diagnostic Guide

What is Komatsu PC850SE-8R Fault Code H-8?

Fault Code H-8 indicates a hydraulic oil temperature sensor circuit malfunction in the Komatsu PC850SE-8R excavator. This code triggers when the Electronic Control Module (ECM) detects an abnormal voltage signal from the hydraulic oil temperature sensor, typically indicating an open circuit, short circuit, or sensor reading outside the normal operating range (usually -40°C to 150°C).

This fault is critical for the PC850SE-8R because hydraulic oil temperature directly affects system viscosity, component lubrication, and overall hydraulic efficiency. The ECM relies on accurate temperature data to protect the hydraulic system from overheating damage and optimize pump performance. When this sensor fails, the machine may enter a protective derate mode or display inaccurate temperature readings, potentially leading to catastrophic hydraulic system failure if ignored.

Common Symptoms

• Hydraulic oil temperature warning light illuminated on the operator display panel
• Machine derate or reduced hydraulic power as the ECM enters protective mode
• Inaccurate or frozen temperature gauge readings (stuck at minimum or maximum values)
• Intermittent fault code that appears and disappears, especially during cold starts or after extended operation
• Audible warning alarms in the cab when the code becomes active

Potential Causes

The H-8 code in used PC850SE-8R excavators typically stems from age-related deterioration:

• Hydraulic oil temperature sensor failure due to thermal cycling and exposure to contaminated hydraulic fluid
• Damaged or corroded wiring harness near the hydraulic tank, where vibration and heat cause insulation breakdown
• Connector corrosion at the sensor plug, particularly common in machines operated in wet or coastal environments
• Chafed wiring at known rub points along the main chassis harness routing near the hydraulic tank
• ECM internal fault (rare) affecting the temperature sensor input circuit
• Contaminated hydraulic oil causing sensor element fouling or coating

How to Troubleshoot and Fix Code H-8

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor mounted on the hydraulic tank. Inspect the sensor connector for corrosion, oil contamination, or loose pins. Check the wiring harness from the sensor to the ECM for visible damage, particularly at bend points and where the harness contacts the frame.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance at various temperatures (consult Komatsu specifications, typically 2,000-3,000 ohms at 20°C). Check for continuity in the wiring harness between the sensor connector and ECM pins. Measure voltage supply from the ECM (usually 5V reference voltage) with the ignition on.

Step 3: Component Replacement and Verification If resistance values are out of specification or wiring shows open/short circuits, replace the faulty component. For used excavators, always clean connector terminals with electrical contact cleaner and apply dielectric grease before reassembly. After repairs, clear the fault code using Komatsu diagnostic software (KOMTRAX) and monitor during a complete heat cycle.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu service manual for your specific serial number and consider professional diagnostic services for complex electrical issues.