Komatsu PC400LC-6 Fault Code H-20: Complete Diagnostic Guide

What is Komatsu PC400LC-6 Fault Code H-20?

Fault Code H-20 indicates a hydraulic oil temperature sensor circuit malfunction in the Komatsu PC400LC-6 excavator. This code is triggered when the Electronic Control Module (ECM) detects an abnormal voltage signal from the hydraulic oil temperature sensor, typically indicating an open circuit, short circuit, or sensor reading outside the normal operating range (usually -40°C to 150°C).

This fault is critical because the hydraulic system's temperature directly affects oil viscosity, component protection, and overall machine performance. The ECM relies on accurate temperature data to prevent overheating damage to pumps, motors, and cylinders. When this sensor fails, the machine may enter a protective derate mode or display inaccurate temperature readings, potentially leading to catastrophic hydraulic system failure if ignored.

Common Symptoms

• Hydraulic oil temperature warning light illuminated on the instrument panel
• Abnormal temperature gauge readings (stuck at minimum, maximum, or erratic fluctuations)
• Reduced hydraulic performance or machine entering protective derate mode
• Audible warning alarm sounding intermittently or continuously
• Machine may operate normally but display persistent warning indicators

Potential Causes

The most common causes of H-20 on used PC400LC-6 excavators include:

• Failed hydraulic oil temperature sensor due to age, thermal cycling, or internal component degradation
• Damaged or corroded wiring harness at connector points, especially near the hydraulic tank where exposure to heat and contamination is highest
• Broken or frayed sensor wires caused by vibration and rubbing against frame components (common wear point near the tank mounting bracket)
• Corroded connector pins at the sensor plug or ECM connection due to moisture intrusion
• ECM internal fault (rare, but possible in high-hour machines)
• Poor ground connection affecting sensor circuit integrity

How to Troubleshoot and Fix Code H-20

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor (typically mounted on the hydraulic tank). Inspect the sensor connector and wiring harness for obvious damage, corrosion, oil contamination, or loose connections. On used excavators, pay special attention to harness routing where wires may rub against metal edges.

Step 2: Electrical Testing Disconnect the sensor and use a digital multimeter to check sensor resistance. At room temperature (20°C), resistance should typically read between 2,000-3,000 ohms (consult your service manual for exact specifications). Test the harness continuity from the sensor connector to the ECM connector—readings should show less than 5 ohms resistance and infinite resistance to ground.

Step 3: Voltage Check With the ignition on and sensor disconnected, measure supply voltage at the harness connector. You should see approximately 5 volts reference voltage from the ECM. If voltage is absent or incorrect, trace the harness for breaks or check ECM power supply circuits.

Step 4: Component Replacement If the sensor tests faulty, replace it with a genuine Komatsu part or quality aftermarket equivalent. Clean all connector pins with electrical contact cleaner and apply dielectric grease to prevent future corrosion. Clear the fault code using Komatsu diagnostic software (KOMTRAX) or a compatible scan tool and verify proper operation.

For used excavators: Always inspect harness condition thoroughly before replacing sensors, as wiring issues account for 60% of temperature sensor faults in high-hour machines.

---

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number and seek professional assistance for complex electrical diagnostics or repairs.