Komatsu PC138US-2 Fault Code H-6: Technical Guide

What is Komatsu PC138US-2 Fault Code H-6?

Fault Code H-6 on the Komatsu PC138US-2 indicates a Hydraulic Oil Temperature Sensor abnormality, specifically detecting an open circuit or short circuit in the hydraulic oil temperature sensor system. This code triggers when the Electronic Control Unit (ECU) receives voltage readings outside the normal operating range (typically 0.5V-4.5V) from the hydraulic oil temperature sensor located in the hydraulic tank.

This fault is critical for the PC138US-2 because the hydraulic system relies on accurate temperature monitoring to prevent oil degradation, protect seals and pumps, and maintain optimal viscosity. When the ECU cannot monitor hydraulic oil temperature, the machine may enter a protective derate mode or restrict certain functions to prevent catastrophic hydraulic system damage.

Common Symptoms

• Amber warning light illuminated on the instrument panel with H-6 displayed on the monitor
• Reduced hydraulic performance or sluggish boom/arm/bucket movements as the system enters protective mode
• Hydraulic system fan running continuously at high speed regardless of actual oil temperature
• Difficulty starting when the hydraulic oil is cold due to improper warm-up protocols
• Error message on the digital display indicating temperature sensor malfunction

Potential Causes

The most common causes for Code H-6 on used PC138US-2 excavators include:

• Damaged or corroded sensor connector at the hydraulic tank, often caused by water intrusion or prolonged exposure to harsh environments
• Broken wiring harness between the sensor and ECU, particularly at rub points near the swing bearing or along the chassis frame
• Failed hydraulic oil temperature sensor (part number 7861-93-3320) due to internal element deterioration common in machines with 5,000+ hours
• Loose or corroded ground connections affecting sensor signal integrity
• Previous owner repairs using incorrect aftermarket sensors with wrong resistance values

How to Troubleshoot and Fix Code H-6

Step 1: Visual Inspection Locate the hydraulic oil temperature sensor on the right side of the hydraulic tank. Inspect the connector for corrosion, bent pins, or oil contamination. Check the wiring harness from the sensor to the ECU for visible damage, particularly where the harness passes through metal grommets or near moving components.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At 68°F (20°C), resistance should read approximately 2.4-2.6 kΩ. If readings show infinite resistance (open circuit) or near-zero resistance (short circuit), replace the sensor.

Step 3: Circuit Verification With the sensor disconnected, measure voltage at the ECU connector side of the harness. You should see approximately 5V reference voltage on the signal wire. If voltage is absent, inspect for broken wires or check the ECU power supply and ground circuits using Komatsu's KOMTRAX diagnostic software or compatible scan tool.

Step 4: Used Equipment Specific Checks On used excavators, pay special attention to corrosion buildup inside connectors and harness chafing at known wear points. Clean all connections with electrical contact cleaner, apply dielectric grease, and secure any loose wiring with proper clamps. Replace any brittle or oil-soaked sections of the harness.

Professional Disclaimer: While this guide provides technical troubleshooting procedures, complex electrical diagnostics should be performed by certified Komatsu technicians with proper diagnostic equipment. Always consult the official service manual and consider professional assistance for used equipment with unknown maintenance history.