What is Komatsu PC120LC-6 Fault Code S-4?

Fault Code S-4 on the Komatsu PC120LC-6 indicates a hydraulic system malfunction, specifically related to the main pump delivery pressure sensor circuit or abnormal pressure detection. This code triggers when the machine's electronic control system detects voltage irregularities, pressure readings outside normal parameters, or a complete signal loss from the pressure sensor monitoring the main hydraulic pump output.

The PC120LC-6 relies on precise hydraulic pressure monitoring to optimize performance and protect critical components. When the ECM (Engine Control Module) cannot accurately read main pump pressure, it may limit machine functions or enter a protective derate mode to prevent catastrophic damage to the hydraulic system. This fault is particularly critical because hydraulic pressure directly affects all excavator operations—from swing and boom movements to attachment functionality.

Common Symptoms

• Hydraulic system warning light illuminated on the instrument panel, often accompanied by an audible alarm
• Reduced hydraulic performance across all functions, with noticeably slower cycle times for boom, arm, and bucket operations
• Intermittent power loss or complete loss of hydraulic functionality during operation
• Machine may enter auto-derate mode, limiting engine RPM to protect the hydraulic system
• Error code display showing S-4 on the monitor panel during startup or operation

Potential Causes

The S-4 code typically results from issues common in used excavators with accumulated operating hours:

• Pressure sensor failure due to internal component degradation or contaminated hydraulic fluid entering the sensor
• Wiring harness damage at known rub points near the main pump assembly, particularly where harnesses contact the swing frame
• Corroded or loose electrical connectors on the pressure sensor circuit, especially on machines exposed to harsh environments
• ECM communication errors caused by voltage fluctuations or poor ground connections
• Hydraulic contamination affecting sensor accuracy, particularly metal particles from worn pump components
• Short circuits in the sensor wiring due to insulation breakdown from heat exposure near the engine

How to Troubleshoot and Fix Code S-4

Step 1: Initial Inspection and Code Verification Connect Komatsu diagnostic software (KOMTRAX or compatible scan tool) to verify the fault code and check freeze frame data. Note operating conditions when the code triggered. Visually inspect the main pump pressure sensor location and wiring harness for obvious damage, oil contamination, or chafing.

Step 2: Electrical Circuit Testing Using a digital multimeter, check the pressure sensor connector for proper voltage supply (typically 5V reference voltage from the ECM). Test ground continuity and inspect for corrosion on connector pins—a common issue on used excavators. Measure sensor output voltage at idle and compare against Komatsu specifications (usually 0.5-4.5V range depending on pressure).

Step 3: Sensor and Harness Evaluation Disconnect the pressure sensor and perform a resistance check according to manufacturer specifications. On used machines, carefully trace the wiring harness from sensor to ECM, checking for worn insulation at pivot points and frame contact areas. Check connector seals for hydraulic fluid intrusion, which degrades electrical connections.

Step 4: Component Replacement and System Verification If sensor values are out of specification or wiring shows damage, replace the faulty component. For used excavators, always replace connector seals and apply dielectric grease to prevent future corrosion. After repairs, clear codes, operate the machine through full hydraulic cycles, and monitor for code recurrence.

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Disclaimer: This guide provides general troubleshooting information. Always consult official Komatsu service manuals and qualified diesel mechanics for complex repairs. Hydraulic system work requires proper training and safety precautions.