Komatsu PC850-8R Fault Code H-27: Complete Diagnostic Guide

What is Komatsu PC850-8R Fault Code H-27?

Fault Code H-27 indicates a hydraulic oil temperature sensor circuit malfunction in the Komatsu PC850-8R excavator. This code is triggered when the Electronic Control Module (ECM) detects an abnormal voltage signal from the hydraulic oil temperature sensor, typically reading outside the normal operating range of 0.5-4.5 volts.

The hydraulic oil temperature sensor is critical for the PC850-8R's performance management system. It monitors the temperature of hydraulic fluid flowing through the main circuit, allowing the ECM to adjust pump output, activate cooling fans, and prevent thermal damage to hydraulic components. When this sensor fails or provides erratic readings, the machine cannot properly regulate hydraulic system temperatures, potentially leading to reduced efficiency, component wear, or catastrophic hydraulic system failure.

Common Symptoms

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

• Warning light illuminated on the monitor panel with "H-27" displayed
• Hydraulic system derate or reduced performance, especially during heavy lifting operations
• Cooling fan running continuously at high speed, regardless of actual oil temperature
• Inaccurate or erratic hydraulic temperature gauge readings on the display
• Potential automatic shutdown if the ECM enters protective mode due to unreliable temperature data

Potential Causes

The most common technical causes for H-27 in used PC850-8R excavators include:

• Sensor failure due to age, thermal cycling, or internal component degradation
• Wiring harness damage at known rub points near the hydraulic tank or along the main boom
• Corroded or loose connector pins at the sensor plug or ECM connection points
• Damaged sensor threads or improper installation causing poor thermal contact with hydraulic fluid
• ECM internal fault (rare) affecting the temperature monitoring circuit
• 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 or main return line). Inspect the wiring harness for obvious damage, abrasion, or oil contamination. Check the connector for corrosion, bent pins, or moisture intrusion—common issues in used excavators operating in harsh environments.

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 for Komatsu NTC sensors). Compare readings against manufacturer specifications. Next, check voltage supply at the harness connector with ignition on—should read approximately 5 volts from the ECM.

Step 3: Circuit Continuity Check Test continuity between the sensor connector and ECM pins to identify potential open circuits or high-resistance connections. Pay special attention to ground circuits, as poor grounding is a frequent culprit in used machinery.

Step 4: Sensor Replacement and Verification If electrical tests confirm sensor failure, replace with a genuine Komatsu sensor (part-specific to PC850-8R). Before installation, clean the mounting boss thoroughly and apply appropriate thread sealant. After replacement, clear codes using Komatsu diagnostic software (KOMTRAX) and verify proper operation under load.

Critical for Used Excavators: Always inspect harness routing for wear points against frame members or hydraulic lines. Replace any abraded sections and secure with proper clamps to prevent recurrence.

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