Komatsu PC800LC-8R Fault Code H-27: Complete Diagnostic Guide

What is Komatsu PC800LC-8R Fault Code H-27?

Fault Code H-27 indicates a hydraulic oil temperature sensor circuit malfunction in the Komatsu PC800LC-8R excavator. This code triggers when the Electronic Control Module (ECM) detects an abnormal voltage signal from the hydraulic oil temperature sensor, typically indicating an open circuit, short circuit, or sensor reading outside the normal operating range (usually -40°C to 150°C).

This fault is critical for the PC800LC-8R because the hydraulic system relies on accurate temperature monitoring to prevent overheating and maintain optimal viscosity. The ECM uses this data to adjust pump output, activate cooling fans, and protect expensive hydraulic components from thermal damage. Ignoring this code can lead to reduced machine performance, accelerated component wear, and potential catastrophic hydraulic system failure.

Common Symptoms

• Hydraulic oil temperature warning light illuminated on the monitor panel
• Reduced hydraulic performance or sluggish operation, especially during warm-up
• Cooling fan running continuously at high speed regardless of actual oil temperature
• Machine derate mode activated, limiting engine RPM and hydraulic flow
• Erratic temperature gauge readings or temperature display showing "---" or error message

Potential Causes

The most common causes of H-27 on used PC800LC-8R excavators include:

• Failed hydraulic oil temperature sensor due to age, thermal cycling, or contamination exposure
• Damaged wiring harness near the hydraulic tank, particularly where cables route along the frame (known rub point on this model)
• Corroded or loose connector pins at the sensor or ECM connection points
• Shorted sensor wiring caused by pinched cables during previous maintenance or repairs
• ECM internal fault (rare, but possible in high-hour machines)
• Contaminated hydraulic oil causing sensor element fouling

How to Troubleshoot and Fix Code H-27

Step 1: Visual Inspection Begin by inspecting the hydraulic oil temperature sensor located on the hydraulic tank. Check for physical damage, oil leaks, or loose mounting. Examine the wiring harness from sensor to ECM, paying special attention to areas where cables contact the frame or pass through grommets—common wear points on used excavators.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At 20°C (68°F), resistance should typically read 2,000-3,000 ohms (consult your service manual for exact specifications). Check for continuity in the wiring harness and verify no shorts to ground. Measure supply voltage at the connector (should be approximately 5V reference voltage from ECM).

Step 3: Connector and Harness Inspection Clean all connector pins with electrical contact cleaner and inspect for corrosion, bent pins, or moisture intrusion. On used machines, check for harness chafing where cables route near moving components. Apply dielectric grease to connections before reassembly.

Step 4: Component Replacement If sensor resistance is out of specification or wiring shows damage, replace the faulty component. Use genuine Komatsu parts or OEM-equivalent sensors to ensure proper ECM communication. After replacement, clear codes using Komatsu diagnostic software (KOMTRAX) and perform a test cycle.

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