Komatsu PC120-6 Fault Code S-6: Complete Diagnostic Guide

What is Komatsu PC120-6 Fault Code S-6?

Fault Code S-6 on the Komatsu PC120-6 excavator indicates a hydraulic system overheat condition, specifically related to the hydraulic oil temperature sensor circuit or excessive hydraulic oil temperature.

This code triggers when the hydraulic oil temperature exceeds the manufacturer's predetermined threshold (typically around 95-100°C or 203-212°F), or when the temperature sensor circuit develops an electrical fault. The PC120-6 uses this monitoring system to protect critical hydraulic components including pumps, motors, and control valves from heat-related damage. Early detection is crucial because sustained high temperatures degrade hydraulic oil properties, accelerate seal deterioration, and can lead to catastrophic pump failure in these work-intensive machines.

Common Symptoms

When Code S-6 is active, operators typically experience:

• Warning light illumination on the instrument panel, often accompanied by an audible alarm
• Reduced hydraulic response or sluggish implement movement as the system enters protective derate mode
• Noticeably hot hydraulic tank or oil reservoir when touched after operation
• Visible smoke or burning odor from the hydraulic tank breather cap in severe cases
• Error code display on the LCD monitor showing "S-6" or "Hydraulic Overheat"

Potential Causes

The most common technical causes for Code S-6 in used PC120-6 excavators include:

• Failed hydraulic oil temperature sensor (thermistor) due to age-related degradation or contamination
• Clogged hydraulic oil cooler fins blocked by debris, reducing heat dissipation efficiency
• Low hydraulic oil level causing insufficient cooling capacity and increased operating temperatures
• Damaged wiring harness at sensor connector points, especially where harness routes near swing motor (common rub point)
• Faulty cooling fan operation including broken fan belts, seized fan bearings, or malfunctioning fan motor
• Contaminated or degraded hydraulic oil with insufficient viscosity grade for operating conditions
• Excessive system pressure from stuck relief valves or binding implements causing heat generation

How to Troubleshoot and Fix Code S-6

Step 1: Visual Inspection Begin by checking the hydraulic oil level using the sight gauge with machine on level ground. Inspect the hydraulic oil cooler for external debris, bent fins, or oil leaks. Examine the cooling fan belt tension and condition, and verify the fan rotates freely without bearing noise.

Step 2: Sensor Circuit Diagnosis Locate the hydraulic temperature sensor (typically mounted on the main control valve or tank). Using a digital multimeter, disconnect the sensor connector and measure resistance across sensor terminals. At 25°C (77°F), resistance should read approximately 2.0-3.0 kΩ. Check for corroded connector pins or damaged wiring—common in used machines with 5,000+ operating hours.

Step 3: Sensor Signal Verification With ignition on (engine off), backprobe the sensor signal wire at the ECM connector. You should read approximately 5V reference voltage. A short to ground (0V) or open circuit (5V constant) confirms wiring issues specific to this model's harness routing under the operator platform.

Step 4: Oil Condition Assessment Sample the hydraulic oil and check for proper ISO VG 46 viscosity grade. In used excavators, oil degradation or wrong viscosity (often from previous owner maintenance errors) causes overheating. Verify oil meets Komatsu specifications and hasn't exceeded recommended change intervals.

Step 5: Cooling System Function Test Start the engine and verify hydraulic fan engagement under load. The PC120-6 uses a hydraulic-driven fan that should increase speed proportionally with hydraulic temperature. Listen for abnormal noise indicating bearing failure or inspect for hydraulic motor leakage at the fan drive circuit.

Disclaimer: This guide provides general diagnostic information. For complex electrical issues or major component replacement, consult a certified Komatsu technician or authorized service center. Always follow manufacturer safety procedures and use proper diagnostic tools when troubleshooting hydraulic systems.