What is Komatsu PC128US-2 Fault Code S-15?

Fault Code S-15 on the Komatsu PC128US-2 indicates a malfunction in the hydraulic oil temperature sensor circuit. This code signals that the Engine Control Module (ECM) has detected an abnormal voltage reading from the sensor monitoring the temperature of the hydraulic oil in the main hydraulic tank.

The hydraulic oil temperature sensor plays a critical role in protecting your excavator's hydraulic system from overheating damage. When hydraulic oil exceeds safe operating temperatures (typically above 180-200°F), the sensor sends signals to the ECM to activate warning systems or potentially limit machine performance. On the PC128US-2, this sensor directly affects system diagnostics and can prevent costly damage to hydraulic pumps, valves, and seals that deteriorate rapidly under excessive heat conditions.

Common Symptoms

• Hydraulic oil temperature warning light illuminated on the instrument panel, even when oil temperature feels normal to touch
• Intermittent gauge readings showing erratic or frozen temperature displays
• Machine may enter derate mode or performance limitation during operation (though less common with S-15)
• ECM logs the fault code in memory, which appears during diagnostic scanning
• In some cases, no noticeable performance issues, but the code prevents passing equipment inspections

Potential Causes

The most common causes of S-15 on used PC128US-2 excavators include:

• Failed hydraulic oil temperature sensor due to internal element deterioration (common after 5,000+ operating hours)
• Damaged or corroded wiring harness connections at the sensor plug, especially where the harness runs near the hydraulic tank mounting bracket
• Chafed wires from vibration contact with the tank or frame—a known wear point on this model
• Corroded or loose ground connections affecting the sensor circuit reference voltage
• ECM internal fault (rare, but possible in machines with 10+ years of service)
• Contaminated hydraulic oil causing sensor coating and false readings

How to Troubleshoot and Fix Code S-15

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor on the main hydraulic tank (typically mounted on the side or top). Inspect the sensor's electrical connector for corrosion, bent pins, or oil contamination. Check the entire wiring harness from sensor to ECM for visible damage, especially where it passes near moving components or sharp edges.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance. At room temperature (68°F), the sensor should read approximately 2,000-3,000 ohms (consult your specific service manual for exact specifications). Test the harness side for continuity to the ECM and check for proper reference voltage (usually 5 volts) with the key in the ON position.

Step 3: Connector and Ground Verification Clean all connector pins with electrical contact cleaner and apply dielectric grease to prevent future corrosion. Verify the sensor's ground circuit has less than 0.5 ohms resistance to chassis ground. On used excavators, ground point corrosion is extremely common and often overlooked.

Step 4: Component Replacement or Further Diagnosis If resistance values are out of specification or the sensor shows physical damage, replace the hydraulic oil temperature sensor with a genuine Komatsu part or OEM equivalent. After replacement, clear the code using Komatsu diagnostic software (KOMTRAX or compatible scan tool) and monitor for recurrence during a complete operating cycle.

Disclaimer: This guide provides general troubleshooting information. Always consult the Komatsu PC128US-2 service manual for model-specific procedures and specifications. If you're uncomfortable performing electrical diagnostics, consult a certified Komatsu technician to prevent further damage to your machine's systems.