Komatsu PC800SE-7 Fault Code H-5: Complete Diagnostic Guide

What is Komatsu PC800SE-7 Fault Code H-5?

Fault Code H-5 indicates a hydraulic oil temperature sensor circuit malfunction in the Komatsu PC800SE-7 excavator. This code is triggered 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 PC800SE-7 because the hydraulic system relies on accurate temperature monitoring to prevent overheating and maintain optimal viscosity. The ECM uses this data to regulate hydraulic pump output, activate cooling fans, and protect expensive hydraulic components from thermal damage. Ignoring this code can lead to reduced machine performance, accelerated component wear, and potential catastrophic hydraulic system failure.

Common Symptoms

When Code H-5 is active, operators typically experience:

• Hydraulic temperature warning light illuminated on the instrument panel
• Reduced hydraulic power or sluggish attachment response, especially during warm-up
• Cooling fan running continuously at high speed regardless of actual oil temperature
• Machine derate mode limiting engine RPM and hydraulic flow to protect the system
• Erratic temperature gauge readings or gauge stuck at minimum/maximum positions

Potential Causes

The most common technical causes for H-5 on used PC800SE-7 excavators include:

• Failed hydraulic oil temperature sensor due to age, thermal cycling, or internal element degradation
• Damaged wiring harness near the hydraulic tank, particularly where cables route along the frame and experience vibration or abrasion
• Corroded or loose connector pins at the sensor or ECM connection points (common in machines exposed to moisture)
• Broken or chafed wires at known rub points near the swing bearing or along the undercarriage
• ECM internal fault (rare, but possible in high-hour machines with electrical system history)

How to Troubleshoot and Fix Code H-5

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor (typically mounted on the hydraulic tank). Inspect the sensor connector for corrosion, bent pins, or moisture intrusion. On used excavators, check the entire wiring harness from sensor to ECM for obvious damage, particularly at flex points and areas where the harness contacts the frame.

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

Step 3: Component Replacement and Verification If the sensor resistance is out of specification or wiring shows open/short circuits, replace the faulty component. For used machines, always apply dielectric grease to connectors to prevent future corrosion. Clear the fault code using Komatsu diagnostic software (KOMTRAX) or a compatible scan tool, then operate the machine through a full heat cycle to verify the repair.

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific serial number and consider professional assistance for complex electrical diagnostics or if you're unfamiliar with high-voltage systems.