Komatsu PC55MR-2 Fault Code S-6: Complete Diagnostic Guide

What is Komatsu PC55MR-2 Fault Code S-6?

Fault Code S-6 on the Komatsu PC55MR-2 indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal engine temperature reading detected by the Electronic Control Module (ECM). This code triggers when the ECM receives voltage signals outside the acceptable range from the coolant temperature sensor, typically indicating either sensor failure, wiring issues, or actual overheating conditions.

This fault is critical for the PC55MR-2 because the engine coolant temperature sensor directly influences fuel injection timing, idle speed control, and engine protection protocols. When this sensor fails, the ECM cannot properly manage engine performance, potentially leading to inefficient combustion, increased emissions, or catastrophic engine damage if actual overheating goes undetected.

Common Symptoms

• Engine warning lamp illuminated on the instrument panel, often accompanied by audible alarm
• Engine running in derate mode (reduced power output) as a protective measure
• Difficulty starting when the engine is cold, or excessive black smoke during warm-up
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Erratic engine idle or rough running, particularly when cold

Potential Causes

The S-6 code on used PC55MR-2 excavators typically stems from these common issues:

• Coolant temperature sensor failure due to age-related degradation or corrosion on sensor terminals
• Damaged wiring harness between the sensor and ECM, particularly at flex points near the engine mounting brackets where vibration causes wire chafing
• Corroded or loose connector pins at the sensor plug, especially on machines operating in wet or salty environments
• Low coolant level causing intermittent sensor exposure and false readings
• ECM ground circuit issues affecting sensor voltage reference
• Previous repairs using incorrect aftermarket sensors with incompatible resistance specifications

How to Troubleshoot and Fix Code S-6

Step 1: Visual Inspection Begin by checking coolant level in the radiator and overflow tank. Inspect the coolant temperature sensor (located on the engine block near the thermostat housing) for physical damage, coolant leaks, or corrosion. On used excavators, carefully examine the wiring harness from the sensor to the ECM, paying special attention to areas where the harness contacts metal surfaces or moving components.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor connector and measure resistance across the sensor terminals. At 68°F (20°C), resistance should typically read 2,000-3,000 ohms (consult your service manual for exact specifications). Test harness continuity from the sensor connector to the ECM pins, and check for shorts to ground. Measure voltage supply at the sensor connector with the key on—you should see approximately 5 volts reference voltage.

Step 3: Component Replacement and Verification If sensor resistance is out of specification or harness damage is found, replace the faulty component using genuine Komatsu parts when possible. For used machines, always apply dielectric grease to connector pins before reassembly to prevent future corrosion. Clear the fault code using Komatsu diagnostic software or by disconnecting the battery for 30 seconds, then operate the machine through a full temperature cycle to verify the repair.

---

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific serial number, and consider professional diagnostic assistance for complex electrical issues or if you're unfamiliar with excavator systems.