Komatsu PC35MR-2 Fault Code S-4: Complete Diagnostic Guide

What is Komatsu PC35MR-2 Fault Code S-4?

Fault Code S-4 on the Komatsu PC35MR-2 indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading. This code triggers when the Electronic Control Module (ECM) detects voltage values outside the acceptable range from the coolant temperature sensor, typically between 0.5V and 4.5V depending on operating temperature.

This is a critical alert for the PC35MR-2's compact diesel engine because incorrect coolant temperature readings can lead to improper fuel injection timing, inefficient combustion, and potential engine overheating. The ECM relies on accurate temperature data to optimize performance and protect engine components in this mini excavator's tight engine bay where heat dissipation is already challenging.

Common Symptoms

When Code S-4 is active on your Komatsu PC35MR-2, you may experience:

• Warning light illuminated on the instrument panel, sometimes accompanied by an audible alarm
• Engine performance issues including rough idling, poor throttle response, or difficulty starting when cold
• Automatic engine derate where the machine limits maximum RPM to prevent potential damage
• Excessive white smoke during startup if the ECM assumes incorrect temperature and over-fuels
• Cooling fan running continuously at high speed regardless of actual engine temperature

Potential Causes

The most common technical failures triggering S-4 in used PC35MR-2 excavators include:

• Coolant temperature sensor failure due to age-related internal resistance drift or element contamination
• Damaged wiring harness near the engine block where vibration causes insulation wear and bare wire contact
• Corroded connector pins at the sensor plug, especially on machines operated in wet or marine environments
• ECM ground circuit issues creating voltage reference problems for sensor readings
• Coolant system problems causing actual overheating that triggers legitimate sensor warnings
• Faulty ECM (rare) with internal circuit board damage affecting the temperature input channel

How to Troubleshoot and Fix Code S-4

Step 1: Visual Inspection Begin by locating the coolant temperature sensor on the cylinder head (driver's side, near the thermostat housing). Inspect the sensor connector for corrosion, moisture intrusion, or pushed-back pins. Check the wiring harness from sensor to ECM for abrasion points where it contacts the engine block or frame—this is extremely common on used excavators with 3,000+ operating hours.

Step 2: Electrical Testing Disconnect the sensor and use a digital multimeter to check resistance across the sensor terminals. At room temperature (68°F/20°C), resistance should read approximately 2,000-3,000 ohms. When warmed to operating temperature (180°F/82°C), resistance should drop to roughly 200-400 ohms. Values outside this range confirm sensor failure.

Step 3: Circuit Verification With the sensor disconnected, check for 5-volt reference voltage at the harness connector using your multimeter with ignition on. Verify ground continuity between the sensor ground pin and battery negative (should read less than 1 ohm resistance). High resistance indicates corroded connections needing cleaning with electrical contact cleaner and dielectric grease application.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Komatsu coolant temperature sensor (Part #6732-81-3140 or equivalent). Aftermarket sensors often have incorrect resistance curves causing repeat codes. Apply thread sealant (not Teflon tape) and torque to 15-18 ft-lbs. Clear codes using Komatsu diagnostic software or compatible scan tool.

For used excavators specifically: Before replacing any components, thoroughly clean all connections and check for coolant leaks that may have damaged electrical components. Inspect the ECM harness connector under the operator's seat for corrosion from water intrusion through worn floor seals.

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Disclaimer: This guide provides general diagnostic information for experienced technicians. Always consult the official Komatsu PC35MR-2 service manual and follow manufacturer specifications. If unfamiliar with electrical diagnostics or engine systems, contact a certified Komatsu technician to prevent equipment damage or safety hazards.