Komatsu PC400-6 Fault Code S-5: Complete Diagnostic Guide

What is Komatsu PC400-6 Fault Code S-5?

Fault Code S-5 on the Komatsu PC400-6 indicates a malfunction in the engine oil pressure sensor circuit or abnormally low engine oil pressure detected by the monitoring system. This code is generated when the Electronic Control Module (ECM) receives signals outside the normal operating parameters from the oil pressure sensor, typically below 0.5 kg/cm² at rated engine speed.

This fault is critical for the PC400-6's 6D125 engine because inadequate oil pressure can lead to catastrophic bearing failure, turbocharger damage, and complete engine seizure. The monitoring system is designed to protect the engine by alerting operators before irreversible damage occurs. In used excavators, this code often appears due to sensor degradation or actual mechanical wear in the lubrication system.

Common Symptoms

When Code S-5 is active, operators typically experience:

• Oil pressure warning light illuminated on the instrument cluster (red oil can symbol)
• Audible alarm sounding continuously during operation
• Engine derate or automatic shutdown in severe cases to prevent damage
• Erratic oil pressure gauge readings or gauge stuck at zero
• Reduced engine performance or hesitation under load due to protective measures

Potential Causes

The most common technical causes for Code S-5 on used PC400-6 excavators include:

• Faulty oil pressure sensor (part #6732-81-3140) due to internal diaphragm failure or contamination
• Damaged wiring harness between the sensor and ECM, particularly at the engine firewall pass-through where vibration causes wire chafing
• Low engine oil level or severely degraded oil viscosity from extended service intervals
• Worn oil pump or pressure relief valve stuck open, common in machines exceeding 8,000 operating hours
• Corroded or loose electrical connectors at the sensor mounting point
• Clogged oil filter causing pressure drop across the lubrication circuit
• ECM calibration issues or internal ECM faults (less common)

How to Troubleshoot and Fix Code S-5

Step 1: Verify Actual Oil Pressure Install a mechanical oil pressure gauge at the sensor port. Run the engine at operating temperature and rated RPM. Normal pressure should read 4.0-5.0 kg/cm². If mechanical pressure is normal, the issue is electrical; if low, investigate mechanical causes first.

Step 2: Inspect the Oil Pressure Sensor and Wiring Disconnect the sensor connector and inspect for corrosion, oil contamination, or bent pins—common issues in used excavators. Check sensor resistance with a multimeter: it should read 240-260 ohms at 20°C. Inspect the wiring harness from sensor to ECM for abrasion, particularly where it contacts the engine block or frame rails.

Step 3: Test Sensor Signal Voltage With ignition on (engine off), backprobe the sensor signal wire. You should see approximately 5V reference voltage from the ECM. During cranking with good oil pressure, voltage should drop to 0.5-4.5V depending on pressure. No voltage change indicates a failed sensor.

Step 4: Check for Mechanical Issues If the sensor tests good but mechanical pressure is low, remove the oil pan and inspect the oil pump pickup screen for debris. Check oil pump gear clearances (should be 0.10-0.20mm). Examine main and rod bearing clearances if the machine has high hours.

Step 5: Clear Code and Monitor After repairs, use Komatsu diagnostic software (KOMTRAX or PC-based tools) to clear the fault code. Operate the machine under load while monitoring live oil pressure data to confirm the repair.

Critical Note for Used Excavators: Before replacing the oil pressure sensor, always inspect connector terminals and harness routing. Vibration-induced wire fatigue at the sensor pigtail is extremely common on PC400-6 models with over 5,000 hours.

---

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number and consider professional diagnosis for complex issues. Improper repairs can result in severe engine damage or safety hazards.