What is Komatsu PC120LC-6 Fault Code S-10?

Fault Code S-10 on the Komatsu PC120LC-6 indicates a hydraulic oil temperature sensor circuit malfunction. This code triggers when the Electronic Control Module (ECM) detects abnormal voltage readings from the hydraulic oil temperature sensor, typically outside the 0.5-4.5V operating range.

The hydraulic oil temperature sensor monitors the working temperature of the hydraulic system, which is critical for the PC120LC-6's performance and longevity. When this sensor fails or sends erratic signals, the ECM cannot properly manage hydraulic system protection protocols, potentially leading to overheating damage to pumps, motors, and seals. For used excavators, this code often appears due to sensor degradation or wiring issues rather than actual overheating conditions.

Common Symptoms

• Warning lamp illuminates on the instrument panel, often accompanied by a temperature gauge malfunction
• Hydraulic system may enter derate mode, limiting operating speed and reducing cycle times
• Erratic temperature gauge readings that fluctuate abnormally or show constant maximum/minimum values
• ECM may log the fault intermittently, especially during machine startup or under load conditions
• In some cases, auxiliary hydraulic functions may experience reduced performance as a protective measure

Potential Causes

The most common technical causes for Code S-10 on used PC120LC-6 excavators include:

• Hydraulic oil temperature sensor failure due to internal resistance drift (common after 5,000+ operating hours)
• Corroded or damaged sensor connector near the hydraulic tank, exposed to moisture and debris
• Wiring harness abrasion at known rub points, particularly where the harness routes along the upper structure near the swing bearing
• Poor ground connection at the ECM or sensor mounting point, causing voltage reference issues
• Contaminated hydraulic oil affecting sensor accuracy (rare but possible in poorly maintained used machines)
• ECM internal fault (least common, but possible in high-hour machines)

How to Troubleshoot and Fix Code S-10

Step 1: Visual Inspection Begin by inspecting the hydraulic oil temperature sensor location (typically on the hydraulic tank). Check the sensor connector for corrosion, bent pins, or moisture intrusion. On used excavators, clean all connector contacts with electrical contact cleaner and apply dielectric grease.

Step 2: Sensor Circuit Testing Using a digital multimeter, disconnect the sensor and measure resistance across the sensor terminals. At 20°C (68°F), resistance should read approximately 2.4-2.8kΩ (consult service manual for exact specifications). Check wiring harness continuity from sensor to ECM connector—resistance should be less than 1Ω. Inspect for voltage drops indicating damaged wiring.

Step 3: Signal Voltage Verification With the sensor connected and ignition on (engine off), backprobe the sensor signal wire at the ECM connector. You should read 0.5-4.5V depending on oil temperature. If voltage reads 0V or battery voltage, suspect open circuit or short to ground/power in the harness.

Step 4: Component Replacement If sensor resistance or voltage is out of specification, replace the hydraulic oil temperature sensor (Komatsu part number 7861-93-3320 or equivalent). For used machines, simultaneously replace the pigtail connector if corrosion is evident. Clear codes using Komatsu diagnostic software (KOMTRAX or compatible scanner) and verify repair.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu PC120LC-6 service manual and consider professional diagnostic assistance for complex electrical issues or if you lack proper diagnostic equipment.