What is Komatsu PC120LC-6 Fault Code H-31?

Komatsu PC120LC-6 fault code H-31 indicates a hydraulic oil temperature sensor circuit malfunction in the excavator's main hydraulic system. This code triggers when the Engine Control Module (ECM) detects abnormal voltage readings from the hydraulic oil temperature sensor, typically outside the normal operating range of 0.5-4.5 volts.

The hydraulic oil temperature sensor plays a critical role in the PC120LC-6's performance management system. It monitors oil temperature to prevent overheating damage to hydraulic components, seals, and the pump. When this sensor malfunctions, the ECM cannot accurately monitor thermal conditions, potentially leading to reduced machine protection and operational efficiency. For used excavators, this code often appears due to sensor degradation or wiring issues common in machines with 5,000+ operating hours.

Common Symptoms

• Hydraulic oil temperature warning light illuminated on the instrument panel
• Reduced hydraulic system performance or sluggish operation as the ECM enters derate mode
• Erratic temperature gauge readings showing unrealistic values (extremely high or low)
• Intermittent code activation during machine startup or operation, especially in older units
• Fan running continuously at high speed regardless of actual oil temperature

Potential Causes

The most common technical causes for H-31 on used PC120LC-6 excavators include:

• Sensor element failure due to prolonged exposure to heat cycling and hydraulic oil contamination
• Wiring harness damage at known rub points near the hydraulic tank and swing motor area
• Corroded or loose electrical connectors at the sensor plug, particularly on machines stored outdoors
• ECM internal fault (less common) affecting the sensor input circuit
• Damaged sensor threads causing improper grounding or oil seepage into the connector

How to Troubleshoot and Fix Code H-31

Step 1: Visual Inspection Locate the hydraulic oil temperature sensor mounted on the hydraulic tank. Inspect the sensor body, wiring harness, and connector for physical damage, oil contamination, corrosion, or loose connections. Check for harness wear at pivot points and metal edges—common failure areas on used machines.

Step 2: Electrical Testing Disconnect the sensor connector. Using a digital multimeter, measure sensor resistance at various temperatures. At 20°C (68°F), resistance should read approximately 2,500 ohms; at 80°C (176°F), approximately 300 ohms. Compare readings to Komatsu specifications. Also check for continuity in the wiring harness between the sensor and ECM connector.

Step 3: Voltage Check With ignition on and sensor disconnected, measure reference voltage at the harness connector. It should read approximately 5 volts. If voltage is absent or incorrect, investigate ECM power supply or internal ECM issues.

Step 4: Replacement and Verification If sensor testing confirms failure, replace with a genuine Komatsu sensor (part number specific to PC120LC-6). Clean connector terminals with electrical contact cleaner before installation. Torque sensor to specifications (typically 15-20 Nm). Clear codes using Komatsu diagnostic software and verify proper operation through a complete heating cycle.

Disclaimer: This guide provides general troubleshooting information. For complex electrical issues or if problems persist, consult a certified Komatsu technician or authorized service center for proper diagnosis and repair.