Komatsu PC400-6 Fault Code H-6: Complete Diagnostic Guide

What is Komatsu PC400-6 Fault Code H-6?

Fault Code H-6 on the Komatsu PC400-6 indicates a hydraulic oil temperature sensor malfunction or abnormally high hydraulic oil temperature. This code is triggered when the Electronic Control Module (ECM) detects either a sensor circuit failure or when hydraulic oil temperature exceeds the safe operating threshold of approximately 110-120°C (230-248°F).

The hydraulic oil temperature sensor monitors the heat level in the hydraulic system, which is critical for maintaining proper viscosity and preventing component damage. On the PC400-6, this sensor feeds data directly to the ECM, which uses it to protect the hydraulic pumps, motors, and control valves from thermal damage. Ignoring this code can lead to accelerated seal wear, reduced hydraulic efficiency, and catastrophic pump failure.

Common Symptoms

When Code H-6 is active, operators typically experience:

• Hydraulic oil temperature warning light illuminated on the instrument panel
• Reduced hydraulic response or sluggish boom/arm/bucket movements as the system enters protective derate mode
• Audible warning alarm sounding in the cab during operation
• Visible steam or excessive heat radiating from the hydraulic tank area
• Automatic machine shutdown in severe overheating conditions to prevent damage

Potential Causes

The most common technical reasons for H-6 code activation on used PC400-6 excavators include:

• Failed hydraulic oil temperature sensor due to age-related degradation or thermal cycling damage
• Damaged sensor wiring harness from rubbing against the hydraulic tank mounting brackets (known wear point on this model)
• Corroded or loose electrical connectors at the sensor or ECM interface
• Actual hydraulic system overheating caused by clogged oil cooler fins, failed cooling fan, or low hydraulic oil level
• Faulty ECM temperature input circuit or internal ECM calibration drift in older machines

How to Troubleshoot and Fix Code H-6

Step 1: Visual Inspection Begin by checking the actual hydraulic oil temperature using an infrared thermometer at the tank. If temperature is normal (below 90°C/194°F), suspect a sensor or electrical issue. Inspect the sensor connector near the hydraulic tank for corrosion, oil contamination, or loose pins—common problems in used excavators.

Step 2: Sensor Circuit Testing Disconnect the temperature sensor connector and measure resistance across the sensor terminals using a digital multimeter. At 25°C (77°F), resistance should read approximately 2,000-3,000 ohms (consult your service manual for exact specifications). If readings are infinite or zero, replace the sensor.

Step 3: Wiring Harness Verification Trace the sensor harness from the tank to the ECM, checking for chafing, cuts, or damage at known rub points near the swing bearing and tank mounting brackets. Use the multimeter to check for continuity and short circuits to ground. On used machines, harness deterioration is extremely common.

Step 4: Hydraulic System Check If sensor and wiring test normal, investigate actual overheating causes: clean the hydraulic oil cooler, verify cooling fan operation, check hydraulic oil level, and inspect for internal pump wear causing excessive heat generation.

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