Komatsu PC35MR-2 Fault Code S-6: Complete Diagnostic Guide

What is Komatsu PC35MR-2 Fault Code S-6?

Fault Code S-6 on the Komatsu PC35MR-2 indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal temperature reading from the engine's cooling system. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects voltage values outside the normal operating range from the coolant temperature sensor, or when the sensor signal is interrupted.

The coolant temperature sensor is critical for the PC35MR-2's engine management system. It monitors engine operating temperature and sends voltage signals to the ECM, which then adjusts fuel injection timing, idle speed, and engine protection protocols. When this sensor fails or provides inaccurate data, the engine cannot properly regulate its thermal operating parameters, potentially leading to overheating, poor fuel economy, or premature engine wear.

Common Symptoms

When fault code S-6 is active on your Komatsu PC35MR-2, you may experience:

• Warning lamp illuminated on the instrument panel, often accompanied by an audible alarm
• Engine running excessively rich or lean, resulting in black or white exhaust smoke
• Hard starting conditions, especially when the engine is cold
• Engine speed limitation or automatic derate mode engaging to prevent overheating damage
• Erratic cooling fan operation, cycling on and off unexpectedly or running continuously

Potential Causes

Several technical failures commonly trigger S-6 on used PC35MR-2 excavators:

• Defective coolant temperature sensor with internal resistance degradation (common after 3,000+ operating hours)
• Damaged wiring harness between the sensor and ECM, particularly at known rub points near the engine mounting brackets
• Corroded or loose electrical connectors at the sensor plug, especially on machines operated in wet or coastal environments
• Faulty ECM with internal circuit board corrosion (less common but possible on older units)
• Actual engine overheating causing legitimate high-temperature readings
• Incorrect coolant level or air pockets in the cooling system affecting sensor accuracy

How to Troubleshoot and Fix Code S-6

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the engine block (typically near the thermostat housing). Check for physical damage, coolant leaks around the sensor base, and examine the wiring harness for abrasion, cuts, or heat damage. On used excavators, inspect connector pins for corrosion or bent terminals—these are common failure points.

Step 2: Electrical Testing Using a digital multimeter, disconnect the coolant sensor connector and measure resistance across the sensor terminals. At room temperature (68°F/20°C), resistance should typically read between 2,000-3,000 ohms. Consult your service manual for exact specifications. Next, check for supply voltage at the harness connector (sensor disconnected)—you should see approximately 5 volts from the ECM.

Step 3: Signal Wire Continuity Test continuity between the sensor connector and the ECM pins using wiring diagrams. Check for shorts to ground or open circuits. Pay special attention to harness routing near moving parts or hot exhaust components where insulation may have degraded over time.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Komatsu or OEM-equivalent temperature sensor. After installation, clear the fault code using Komatsu diagnostic software (KOMTRAX compatible tool) or a compatible scan tool. Monitor coolant temperature readings during a test run to verify proper operation.

Important for Used Equipment: Before replacing the sensor, verify actual coolant condition and system pressure. Contaminated coolant or thermostat failures can cause legitimate overheating that triggers S-6.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu PC35MR-2 service manual and consider professional technician assistance for complex electrical diagnostics or if you're unfamiliar with high-voltage systems.