Komatsu PC800-8R Fault Code H-27: Complete Diagnostic Guide

What is Komatsu PC800-8R Fault Code H-27?

Fault Code H-27 indicates a hydraulic oil temperature sensor circuit malfunction in the Komatsu PC800-8R 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 failure.

The hydraulic oil temperature sensor is critical for monitoring the operating temperature of the hydraulic system. This data allows the ECM to protect the machine from overheating damage by adjusting pump output or triggering warning systems. On the PC800-8R, maintaining proper hydraulic temperature is essential for optimal performance of the large-capacity hydraulic pumps and preventing premature seal and component failure.

Common Symptoms

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

• Hydraulic oil temperature warning light illuminated on the monitor panel
• Inaccurate or erratic temperature readings displayed (may show extremely high or low values)
• Reduced hydraulic performance or automatic derate mode activation as a protective measure
• Intermittent alarm buzzer activation, especially during warm-up or heavy operation
• Potential activation of backup mode, limiting machine functions to prevent thermal damage

Potential Causes

The most common technical causes for H-27 on used PC800-8R excavators include:

• Failed hydraulic oil temperature sensor due to age, thermal cycling, or internal component degradation
• Damaged or corroded wiring harness connections at the sensor plug (common wear point near hydraulic tank)
• Chafed wiring where the harness routes along the frame or crosses moving components
• Corroded or loose connector pins at the sensor or ECM interface
• ECM internal fault (rare, but possible in high-hour machines)
• Contaminated hydraulic oil causing sensor element fouling or coating

How to Troubleshoot and Fix Code H-27

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor (typically mounted on the hydraulic tank). Inspect the sensor connector for corrosion, oil contamination, bent pins, or moisture intrusion. On used excavators, check the entire wiring harness route for abrasion points, especially where it contacts frame members or moving components.

Step 2: Electrical Testing Disconnect the sensor and use a digital multimeter to measure resistance across the sensor terminals. Compare readings against Komatsu specifications (typically 200-400 ohms at 20°C, varying with temperature). Check for continuity in the wiring harness from sensor to ECM. Measure voltage supply at the connector (should be approximately 5V reference voltage from ECM).

Step 3: Component Replacement and Verification If sensor resistance is out of specification or wiring shows damage, replace the faulty component. Clean all connector terminals with electrical contact cleaner before reassembly. After repairs, clear the fault code using Komatsu diagnostic software (KOMTRAX) or a compatible scan tool, then operate the machine through a full temperature cycle to verify the fix.

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