Komatsu PC850-8R Fault Code H-8: Complete Diagnostic Guide

What is Komatsu PC850-8R Fault Code H-8?

Fault Code H-8 on the Komatsu PC850-8R indicates a hydraulic oil temperature sensor circuit malfunction or abnormally high hydraulic oil temperature. This code is triggered when the machine's Electronic Control Module (ECM) detects either an electrical fault in the temperature sensor circuit or when hydraulic oil temperature exceeds the manufacturer's safe operating threshold (typically above 110-120°C or 230-248°F).

This fault is critical for the PC850-8R because excessive hydraulic oil temperature can cause rapid degradation of seals, accelerated oil breakdown, and reduced hydraulic system efficiency. The hydraulic temperature sensor monitors oil temperature in the main hydraulic tank and sends voltage signals to the ECM. Any disruption in this circuit or genuine overheating conditions will trigger H-8, potentially limiting machine performance to prevent catastrophic system damage.

Common Symptoms

When Code H-8 is active, operators typically experience:

• Warning light or alarm on the monitor panel, often accompanied by an audible buzzer
• Reduced hydraulic response or sluggish boom/arm/bucket movements as the system enters derate mode
• Visible temperature gauge reading in the red zone on the instrument cluster
• Automatic engine speed reduction to protect hydraulic components from thermal damage
• Hot hydraulic oil smell or visible heat shimmer near the hydraulic tank (in genuine overheating cases)

Potential Causes

The most common technical causes for H-8 on used PC850-8R excavators include:

• Faulty hydraulic oil temperature sensor – sensor element degradation is common after 8,000+ operating hours
• Damaged or corroded wiring harness – particularly at connector points near the hydraulic tank where vibration and heat exposure are constant
• Genuine hydraulic system overheating – caused by clogged hydraulic oil cooler fins, low coolant flow, or malfunctioning cooling fan
• Blocked hydraulic return filters creating excessive system backpressure and heat generation
• Low hydraulic oil level reducing heat dissipation capacity
• ECM calibration issues or software glitches (less common but possible in older machines)

How to Troubleshoot and Fix Code H-8

Step 1: Visual Inspection Begin by checking the actual hydraulic oil temperature. Feel the hydraulic tank exterior – if it's uncomfortably hot to touch, you have genuine overheating. Inspect the hydraulic oil cooler for debris, bent fins, or blockages. On used excavators, accumulated dirt and debris are frequent culprits. Check hydraulic oil level using the sight glass.

Step 2: Electrical Circuit Testing Locate the hydraulic temperature sensor (typically mounted on the side of the hydraulic tank). Disconnect the sensor connector and inspect for corrosion, oil contamination, or damaged pins – common issues on machines with 5,000+ hours. Using a digital multimeter, measure sensor resistance. At room temperature (20°C/68°F), resistance should typically read 2,000-3,000 ohms (consult your service manual for exact specifications). Test harness continuity from sensor connector to ECM – look for resistance values under 5 ohms.

Step 3: Sensor and Component Replacement If sensor resistance is out of specification or the harness shows opens/shorts, replace the faulty component. For used excavators, always inspect harness routing for rub points against frame members or hydraulic lines – a common wear point on the PC850-8R. If electrical circuits test normal but overheating persists, clean the hydraulic cooler thoroughly, replace hydraulic filters, and verify cooling fan operation.

Step 4: System Verification Clear the fault code using Komatsu diagnostic software (KOMTRAX) or a compatible scan tool. Operate the machine under load while monitoring hydraulic temperature. The code should not return if repairs were successful.

---

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number and consider professional assistance from certified Komatsu technicians for complex repairs.