Komatsu PC850SE-8R Fault Code H-27: Complete Diagnostic Guide

What is Komatsu PC850SE-8R Fault Code H-27?

Fault Code H-27 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, which monitors the operating temperature of the hydraulic system fluid.

The hydraulic oil temperature sensor plays a critical role in protecting the PC850SE-8R's hydraulic system from overheating damage. When functioning properly, it allows the ECM to implement protective measures—such as reducing engine power or activating warning systems—before hydraulic oil reaches temperatures that could degrade seals, damage pumps, or reduce system efficiency. For a machine of this size and complexity, maintaining proper hydraulic oil temperature is essential for optimal performance and component longevity.

Common Symptoms

When Code H-27 is active, operators typically experience:

• Hydraulic oil temperature warning light illuminated on the instrument panel
• Reduced machine performance or derated engine power as a protective measure
• Inaccurate or erratic hydraulic oil temperature readings on the monitor display
• ECM may default to a predetermined temperature value, potentially causing unnecessary warnings
• Intermittent code activation, especially during machine warm-up or under heavy loads

Potential Causes

The most common technical reasons for H-27 in used PC850SE-8R excavators include:

• Faulty hydraulic oil temperature sensor due to age-related degradation or thermal cycling damage
• Damaged or corroded wiring harness connections at the sensor plug, particularly from exposure to hydraulic fluid leaks
• Broken or chafed sensor wiring along routing points near the hydraulic tank or frame rails (common wear areas on used machines)
• Poor ground connection at the sensor circuit or ECM ground points
• ECM internal fault (rare, but possible in high-hour machines)
• Contamination or moisture intrusion in the sensor connector

How to Troubleshoot and Fix Code H-27

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor, typically mounted on or near the hydraulic oil tank. Inspect the sensor connector for corrosion, oil contamination, bent pins, or moisture. On used excavators, check the wiring harness routing for signs of rubbing against frame components, heat damage from nearby hydraulic lines, or previous repair attempts.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance at various temperatures (consult Komatsu specifications, typically 200-400 ohms at 20°C). Check voltage supply at the harness connector with ignition on (should read approximately 5 volts). Verify ground circuit continuity between the sensor ground pin and ECM ground.

Step 3: Harness and Connection Verification Inspect the entire sensor wiring harness from sensor to ECM, paying special attention to areas where the harness passes through bulkheads or near moving components. For used machines, corrosion at splice points or connector terminals is common. Clean all connections with electrical contact cleaner and apply dielectric grease.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Komatsu or OEM-equivalent sensor. After replacement, clear codes using Komatsu diagnostic software (KOMTRAX or compatible scan tool) and verify proper operation under normal operating conditions.

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