Komatsu PC55MR-2 Fault Code H-5: Complete Diagnostic Guide

What is Komatsu PC55MR-2 Fault Code H-5?

Komatsu PC55MR-2 Fault Code H-5 indicates a hydraulic oil temperature sensor malfunction or abnormally high hydraulic oil temperature in the machine's hydraulic system. This code is triggered when the Engine Control Module (ECM) detects either a sensor circuit failure or when hydraulic oil temperature exceeds the manufacturer's safe operating threshold (typically above 95-100°C/203-212°F).

This fault is critical for the PC55MR-2 because excessive hydraulic temperatures can damage seals, hoses, and hydraulic pump components, while sensor failures prevent proper temperature monitoring. The hydraulic system is the lifeblood of excavator operation, and maintaining proper oil temperature ensures optimal viscosity, component longevity, and safe machine performance.

Common Symptoms

When Code H-5 is active, operators typically experience:

• Warning light or alarm illuminated on the instrument cluster, often accompanied by an audible buzzer
• Reduced hydraulic response or sluggish operation of boom, arm, and bucket functions
• Automatic derate mode where the engine reduces power output to protect hydraulic components
• Visible smoke or burning smell from the hydraulic tank area in severe overheating cases
• Erratic temperature gauge readings or gauge stuck at maximum reading

Potential Causes

The most common technical reasons for H-5 fault codes on used PC55MR-2 excavators include:

• Faulty hydraulic oil temperature sensor due to age-related degradation or contamination exposure
• Damaged or corroded wiring harness at connector points near the hydraulic tank (common wear point on used machines)
• Actual hydraulic oil overheating caused by plugged oil cooler fins, low coolant levels, or malfunctioning cooling fan
• Contaminated or degraded hydraulic oil with incorrect viscosity reducing heat dissipation efficiency
• Shorted or open circuit in the sensor wiring, particularly where harnesses rub against frame components
• Failed ECM input circuit for temperature monitoring (less common but possible in high-hour machines)

How to Troubleshoot and Fix Code H-5

Step 1: Visual Inspection and Initial Checks Start by inspecting the hydraulic oil temperature sensor located on the hydraulic tank. Check the wiring harness for visible damage, particularly at connector pins and where the harness routes near moving components. On used excavators, corrosion buildup at connector terminals is extremely common—clean with electrical contact cleaner and examine for green corrosion deposits.

Step 2: Sensor Circuit Testing Using a digital multimeter, disconnect the temperature sensor connector and measure resistance across the sensor terminals. Compare readings against Komatsu specifications (typically 200-300 ohms at 20°C ambient temperature). Test the wiring harness continuity from sensor to ECM connector. Check for short circuits to ground by measuring resistance between each wire and machine chassis (should read infinite resistance).

Step 3: Verify Actual Hydraulic Temperature Use an infrared thermometer to measure actual hydraulic oil temperature at the tank. If oil temperature exceeds 85°C (185°F) during normal operation, inspect the hydraulic oil cooler for debris blockage, verify cooling fan operation, and check hydraulic oil level and condition. On used machines, internal cooler tube corrosion can significantly reduce cooling efficiency.

Step 4: Component Replacement and System Reset If sensor resistance is out of specification or wiring shows damage, replace the hydraulic oil temperature sensor (Komatsu part number specific to PC55MR-2). After repairs, clear fault codes using Komatsu diagnostic software or by disconnecting the battery negative terminal for 5 minutes. Operate the machine through a full work cycle and monitor for code recurrence.

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu PC55MR-2 service manual and consider professional technician assistance for complex electrical diagnostics or if you're unfamiliar with hydraulic systems. Improper repairs can result in equipment damage or safety hazards.