Komatsu PC55MR-2 Fault Code H-8: Complete Diagnostic Guide

What is Komatsu PC55MR-2 Fault Code H-8?

Fault Code H-8 on the Komatsu PC55MR-2 indicates a hydraulic oil temperature sensor malfunction or abnormal hydraulic system overheating condition. This code is triggered when the machine's Electronic Control Module (ECM) detects either an irregular signal from the hydraulic oil temperature sensor or when hydraulic fluid temperatures exceed the manufacturer's safe operating threshold (typically above 95-100°C or 203-212°F).

This code is critical because the PC55MR-2's compact hydraulic system relies on precise temperature monitoring to prevent catastrophic damage to hydraulic pumps, control valves, and cylinder seals. Prolonged operation with elevated hydraulic temperatures can lead to rapid oil degradation, reduced viscosity, and complete hydraulic system failure.

Common Symptoms

• Flashing warning light on the instrument panel (hydraulic temperature icon)
• Reduced hydraulic response or sluggish boom/arm/bucket movements
• Automatic engine derating to lower RPM to protect the system
• Audible alarm from the cab (continuous beeping or buzzing)
• Visible smoke or burning smell from the hydraulic tank area in severe cases

Potential Causes

The H-8 code in used PC55MR-2 excavators typically stems from these issues:

• Faulty hydraulic oil temperature sensor (common failure point after 3,000+ operating hours)
• Damaged or corroded sensor wiring harness near the hydraulic tank mounting bracket (known rub point on this model)
• Clogged hydraulic oil cooler fins restricting airflow
• Low hydraulic oil level causing inadequate cooling circulation
• Failing hydraulic cooling fan or damaged fan belt
• Degraded hydraulic oil with contaminated or wrong viscosity grade
• Short circuit or open circuit in sensor connector terminals (especially in machines exposed to moisture)

How to Troubleshoot and Fix Code H-8

Step 1: Visual Inspection Check hydraulic oil level using the sight glass—top off if low. Inspect the hydraulic oil cooler for debris blockage; clean fins with compressed air. Examine the cooling fan belt for proper tension and wear.

Step 2: Sensor and Wiring Check Locate the hydraulic temperature sensor on the left side of the hydraulic tank. Disconnect the two-pin connector and inspect for corrosion, bent pins, or moisture intrusion. On used machines, check the wiring harness from sensor to ECM for abrasion points near the swing motor mounting bracket—a common wear location on PC55MR-2 models.

Step 3: Electrical Testing Using a digital multimeter, measure sensor resistance. At room temperature (20°C), resistance should be approximately 2,000-3,000 ohms (consult service manual for exact specs). Check for continuity in the wiring harness—any infinite resistance indicates an open circuit requiring harness repair or replacement.

Step 4: Temperature Verification If sensor tests good electrically, use an infrared thermometer to verify actual hydraulic oil temperature against the sensor reading displayed on the Komatsu diagnostic software (KOMTRAX or PC-based tool). A variance greater than 10°C suggests sensor drift requiring replacement.

Step 5: ECM Code Clearing After repairs, clear the fault code using diagnostic software or by disconnecting the battery negative terminal for 5 minutes. Test operate the machine under load to verify repair.

For used excavators, always replace the hydraulic oil and filter if contamination is suspected, as degraded oil accelerates sensor failure.

---

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 when warranty coverage applies.