Komatsu PC850SE-8R Fault Code H-20: Complete Diagnostic Guide

What is Komatsu PC850SE-8R Fault Code H-20?

Fault Code H-20 indicates a hydraulic oil temperature sensor circuit malfunction in the Komatsu PC850SE-8R excavator. This code is triggered 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 monitor fluid temperatures and activate warning systems or reduce machine performance if temperatures exceed safe operating limits. For a machine of this size and complexity, maintaining proper hydraulic oil temperature is essential for preventing catastrophic component failure and ensuring optimal performance of all hydraulic functions.

Common Symptoms

When Code H-20 is active on your PC850SE-8R, operators typically experience:

• Hydraulic oil temperature warning light illuminated on the monitor panel
• Inaccurate or erratic temperature readings displayed on the operator console
• Machine derate mode activation, limiting engine RPM and hydraulic flow
• Inability to clear the fault code even after machine cool-down periods
• Potential activation of backup/limp mode, restricting excavator performance to protect components

Potential Causes

The most common technical reasons for H-20 fault code activation include:

• Failed hydraulic oil temperature sensor due to internal component degradation (common in high-hour used machines)
• Damaged or corroded wiring harness connections at the sensor plug
• Harness chafing at known rub points near the hydraulic tank mounting area
• Contaminated or corroded sensor connector pins from hydraulic oil seepage
• ECM internal fault or corrupted calibration data (less common)
• Broken or shorted wiring between the sensor and ECM
• Improper sensor installation or loose mounting after previous maintenance

How to Troubleshoot and Fix Code H-20

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 routing for signs of chafing, especially where it contacts frame members or hydraulic lines.

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 harness continuity from the sensor connector back to the ECM. Measure supply voltage at the sensor connector with ignition on (should read approximately 5 volts).

Step 3: Component Replacement and Verification If sensor resistance is out of specification or harness shows open circuits or short to ground, replace the faulty component. For used machines, always apply dielectric grease to connector pins 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 work cycle to verify repair.

Critical for Used Equipment: Before replacing the sensor, thoroughly clean all connections and check for harness wear patterns specific to the PC850SE-8R model, particularly near hydraulic pump mounting points.

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Disclaimer: This guide provides general diagnostic information. Always consult Komatsu service manuals and qualified technicians for complex repairs. Improper diagnostics may lead to additional component damage or safety hazards.