Komatsu PC138US-2 Fault Code S-6: Complete Diagnostic Guide

What is Komatsu PC138US-2 Fault Code S-6?

Fault Code S-6 on the Komatsu PC138US-2 indicates a hydraulic oil temperature sensor circuit malfunction or excessively high hydraulic oil temperature. This code triggers when the Electronic Control Module (ECM) detects either an abnormal voltage signal from the hydraulic oil temperature sensor or when the measured temperature exceeds the manufacturer's safe operating threshold (typically above 248°F/120°C).

This diagnostic trouble code is critical for the PC138US-2 because excessive hydraulic oil temperature can cause accelerated seal degradation, reduced hydraulic efficiency, and potential catastrophic pump failure. The monitoring system protects the machine's variable displacement piston pumps and control valves from thermal damage that commonly occurs in used excavators operating under heavy loads.

Common Symptoms

When Code S-6 activates on your Komatsu PC138US-2, operators typically experience:

• Illuminated warning lamp on the instrument panel with audible alarm
• Reduced hydraulic power or sluggish boom/arm/bucket response as the system enters derate mode
• Physically hot hydraulic tank surface or burning oil smell from the engine compartment
• Loss of auxiliary hydraulic functions or complete system shutdown in severe cases
• Intermittent code activation during high ambient temperatures or continuous heavy operation

Potential Causes

The most common technical reasons for S-6 codes in used PC138US-2 excavators include:

• Faulty hydraulic oil temperature sensor (internal resistance drift common after 5,000+ hours)
• Damaged or corroded wiring harness between sensor and ECM, particularly at routing points near the hydraulic tank
• Genuinely overheated hydraulic oil due to clogged hydraulic oil cooler fins or malfunctioning cooling fan
• Degraded hydraulic oil with wrong viscosity grade reducing heat dissipation efficiency
• ECM calibration issues or ground circuit failures causing false temperature readings
• Restricted hydraulic return filters forcing increased system pressure and heat generation

How to Troubleshoot and Fix Code S-6

Step 1: Verify Actual Hydraulic Temperature Use an infrared thermometer to measure actual hydraulic tank temperature. Compare against the ECM's displayed reading via Komatsu diagnostic software (KOMTRAX or aftermarket tools). A variance exceeding 15°F indicates sensor or wiring issues rather than genuine overheating.

Step 2: Inspect Sensor and Harness Located on the hydraulic tank, examine the temperature sensor connector for corrosion, bent pins, or oil contamination. Check harness routing for rub-through damage against frame components—common on used machines. Measure sensor resistance (should read approximately 200-300 ohms at 77°F/25°C; consult service manual for exact specifications).

Step 3: Test Sensor Circuit Voltage With ignition on but engine off, backprobe the sensor connector. The ECM should supply approximately 5V reference voltage. Resistance out of specification or open/short circuits require sensor replacement (Komatsu part number specific to PC138US-2).

Step 4: Address Genuine Overheating If temperature is legitimately high, clean the hydraulic oil cooler, verify cooling fan operation, check hydraulic oil level and condition, and replace return filters. For used excavators, consider hydraulic oil analysis to detect internal pump wear generating excessive heat.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu service manual for your specific serial number and seek qualified technician assistance for complex electrical diagnostics or hydraulic system repairs.