Komatsu PC120-6 Fault Code S-10: Complete Diagnostic Guide

What is Komatsu PC120-6 Fault Code S-10?

Fault Code S-10 indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal temperature reading detected by the Engine Control Module (ECM). This diagnostic trouble code specifically monitors the coolant temperature sensor signal integrity and range on Komatsu's PC120-6 excavator model.

The ECM continuously monitors coolant temperature to optimize fuel injection timing, control engine warm-up cycles, and prevent overheating damage. On the PC120-6's SAA4D102E-1 diesel engine, this sensor is critical for maintaining proper combustion efficiency and protecting engine components. When the sensor signal falls outside manufacturer-specified parameters (typically 0.5-4.5V), the S-10 code triggers, potentially causing the engine to enter a protective derate mode.

Common Symptoms

• Warning lamp illumination on the instrument panel (engine caution light)
• Engine hard starting or rough idle, especially when cold
• Black smoke during acceleration due to improper fuel-air mixture
• Reduced engine power or derate condition limiting maximum RPM
• Cooling fan running continuously at high speed regardless of actual temperature

Potential Causes

The most frequent causes of S-10 on used PC120-6 excavators include:

• Coolant temperature sensor degradation – common failure after 5,000+ operating hours due to thermal cycling
• Wiring harness chafing at the sensor connector near the thermostat housing (known wear point on this model)
• Corroded or loose connector pins at the sensor plug, especially on machines operated in wet or coastal environments
• ECM internal circuit failure – rare but documented in PC120-6 models with serial numbers below 52000
• Contaminated coolant affecting sensor accuracy through mineral deposit buildup

How to Troubleshoot and Fix Code S-10

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the front left side of the cylinder head. Check for coolant leaks, damaged wiring, or connector corrosion. On used excavators, examine the entire wire harness from sensor to ECM for rub-through points against metal brackets or engine components.

Step 2: Electrical Testing Using a digital multimeter, perform resistance checks. Disconnect the sensor and measure resistance across terminals – should read approximately 2.5kΩ at 20°C. Verify supply voltage at the harness connector (ECM side) reads 5V ±0.25V. Check for continuity and short-to-ground conditions in the harness wiring.

Step 3: Component Replacement If sensor resistance is out of specification or harness shows damage, replace the affected component using genuine Komatsu part 6732-81-3140. Before installation, clean the sensor mounting bore and apply fresh thread sealant. After replacement, clear codes using Komatsu diagnostic software (KOMTRAX) and verify proper operation.

For used machines, always inspect connector seals and apply dielectric grease to prevent future corrosion-related failures.

---

Disclaimer: This guide provides general diagnostic information. Complex electrical issues may require professional diagnosis with OEM-specific tools and trained technicians familiar with Komatsu systems.