Komatsu D85PX-15 Fault Code H-6: Complete Diagnostic Guide

What is Komatsu D85PX-15 Fault Code H-6?

Fault Code H-6 indicates a hydraulic oil temperature sensor malfunction or abnormally high hydraulic oil temperature in the Komatsu D85PX-15 bulldozer. This code is triggered when the Engine Control Module (ECM) detects either an electrical fault in the temperature sensor circuit or when the hydraulic oil exceeds the manufacturer's specified safe operating temperature threshold (typically above 248°F/120°C).

This fault is critical for the D85PX-15 because excessive hydraulic temperatures can cause rapid degradation of seals, accelerated fluid oxidation, and reduced hydraulic component life. The hydraulic system powers all blade, ripper, and steering functions, making this code a priority issue that can lead to costly component failures if ignored.

Common Symptoms

When Code H-6 is active on your D85PX-15, operators typically experience:

• Hydraulic oil temperature warning light illuminated on the instrument cluster (red or amber depending on severity)
• Noticeably sluggish hydraulic response during blade or ripper operation, especially under load
• Automatic engine derate or speed limitation to protect hydraulic components
• Audible warning alarm from the cab monitoring system
• Physical signs like excessive heat radiating from the hydraulic tank or visible fluid discoloration (darkened or burnt smell)

Potential Causes

In used D85PX-15 machines, Code H-6 commonly stems from:

• Failed hydraulic oil temperature sensor (resistance drift or internal short circuit)
• Damaged wiring harness between the sensor and ECM, particularly at known rub points near the hydraulic tank mounting bracket
• Corroded or loose connector terminals at the sensor plug (common in machines operating in wet/muddy conditions)
• Actual hydraulic overheating due to plugged hydraulic oil cooler fins or malfunctioning cooling fan
• Low hydraulic oil level causing insufficient cooling capacity
• Worn hydraulic pump generating excessive heat through internal leakage

How to Troubleshoot and Fix Code H-6

Step 1: Visual Inspection Check the hydraulic oil level using the sight glass when the machine is cold. Inspect the hydraulic oil cooler for debris buildup, bent fins, or oil leaks. Examine the temperature sensor (located on the hydraulic tank) for physical damage or oil contamination around the connector.

Step 2: Electrical Testing Disconnect the temperature sensor connector and use a digital multimeter to measure resistance. At 68°F (20°C), resistance should typically read 2,000-3,000 ohms (consult your service manual for exact specs). Test the wiring harness for continuity between the sensor connector and ECM pins, checking for shorts to ground.

Step 3: Connector and Harness Inspection (Critical for Used Machines) Carefully inspect the sensor connector for green corrosion, pushed-back pins, or moisture intrusion. On used D85PX-15s, check the harness routing near the hydraulic tank where vibration causes insulation wear. Apply dielectric grease to clean connections before reassembly.

Step 4: Component Replacement or System Service Replace the temperature sensor if resistance values are out of specification or if physical damage is evident. If electrical testing passes, service the hydraulic system: change contaminated oil, clean the cooler, and verify fan operation using Komatsu diagnostic software (KOMTRAX) to monitor real-time temperatures.

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu D85PX-15 service manual and consider professional evaluation for complex hydraulic system issues, especially on high-hour used machines.