What is Komatsu PC120-6 Fault Code H-6?

Fault Code H-6 on the Komatsu PC120-6 excavator indicates a hydraulic oil temperature sensor malfunction or abnormally high hydraulic oil temperature. This code is triggered when the Electronic Control Module (ECM) detects that hydraulic fluid temperature exceeds the manufacturer's safe operating threshold (typically above 110°C/230°F) or when the temperature sensor circuit experiences electrical irregularities.

The hydraulic system is the lifeblood of any excavator, and maintaining proper oil temperature is critical for viscosity control, seal integrity, and component longevity. On the PC120-6, this sensor directly influences pump output regulation and implements emergency protection protocols. Ignoring this code can lead to accelerated wear on hydraulic pumps, control valves, and cylinder seals, potentially causing catastrophic system failure.

Common Symptoms

• Reduced hydraulic power or sluggish operation of boom, arm, and bucket functions
• Illuminated warning light on the instrument panel (typically an oil temperature icon)
• Unusual hydraulic system noise, including whining or cavitation sounds from pumps
• Intermittent loss of hydraulic response during extended work cycles
• Automatic machine derate or shutdown in severe overheat conditions

Potential Causes

On used PC120-6 excavators, several age-related and maintenance issues commonly trigger Code H-6:

• Defective hydraulic oil temperature sensor due to prolonged exposure to heat cycles (sensor resistance drift)
• Corroded or damaged wiring harness at connector pins near the hydraulic tank, especially where harnesses contact frame edges
• Actual hydraulic oil overheating caused by restricted oil cooler fins (debris accumulation), failed cooling fan, or low coolant levels
• Contaminated hydraulic oil with improper viscosity reducing heat dissipation efficiency
• Worn hydraulic pump generating excessive heat due to internal leakage
• ECM ground faults or poor chassis grounding connections

How to Troubleshoot and Fix Code H-6

Step 1: Visual Inspection Begin with a thorough examination of the hydraulic oil cooler. Remove debris, mud, or vegetation blocking airflow through cooling fins. Check hydraulic oil level and condition—milky appearance indicates water contamination; dark, burnt smell suggests thermal breakdown requiring immediate oil change.

Step 2: Sensor Circuit Testing Locate the hydraulic temperature sensor (typically mounted on the hydraulic tank). Disconnect the sensor harness connector and inspect for corrosion, bent pins, or oil intrusion. Using a digital multimeter, measure sensor resistance at ambient temperature (should read approximately 2-3kΩ at 20°C; consult service manual for exact specifications). Compare against manufacturer resistance-temperature chart. Also check harness continuity to the ECM and verify proper ground connection.

Step 3: Live Temperature Verification If sensor readings are within specification, use an infrared thermometer to measure actual hydraulic tank temperature during operation. Compare against sensor-reported temperature using Komatsu diagnostic software (PC-compatible diagnostic tools). A variance greater than 10°C indicates sensor drift requiring replacement.

Step 4: System Performance Check For used excavators, inspect hydraulic pump case drain flow—excessive flow indicates internal wear generating heat. Check relief valve settings and inspect all cylinder rod seals for bypass issues causing system inefficiency.

Critical for Used Equipment: Before replacing the temperature sensor (Part #: 7861-93-1651), thoroughly inspect the wiring harness routing from sensor to ECM. On older PC120-6 models, harnesses often contact sharp edges near the hydraulic tank mounting bracket, causing intermittent shorts. Apply dielectric grease to connectors and secure harnesses with new clamps away from friction points.

---

Disclaimer: This guide provides general troubleshooting information. Hydraulic systems operate under extreme pressure and temperature. Always consult the official Komatsu PC120-6 service manual and consider professional diagnostics for complex electrical or hydraulic repairs. Improper repairs may void warranties or create safety hazards.