Komatsu PC55MR-2 Fault Code H-27: Complete Diagnostic Guide

What is Komatsu PC55MR-2 Fault Code H-27?

Fault Code H-27 indicates a malfunction in the hydraulic oil temperature sensor circuit, specifically detecting an abnormal voltage signal or open/short circuit condition in the temperature monitoring system. This code is triggered when the ECM (Engine Control Module) receives sensor readings outside the normal operating range of 0.5-4.5 volts, suggesting either sensor failure or wiring issues.

This fault is critical for the PC55MR-2 because hydraulic oil temperature directly affects system efficiency, component longevity, and operational safety. When undetected, overheating can cause accelerated wear on hydraulic pumps, seals, and control valves, potentially leading to costly repairs on used machines already experiencing age-related degradation.

Common Symptoms

• Warning light illuminated on the instrument panel (typically a hydraulic system indicator or general caution light)
• Erratic temperature gauge readings or gauge stuck at minimum/maximum positions
• Reduced hydraulic performance as the system may enter a protective derate mode
• Cooling fan running continuously regardless of actual oil temperature
• Sluggish auxiliary function response during initial startup or under load conditions

Potential Causes

The most common technical reasons for H-27 activation on used PC55MR-2 excavators include:

• Hydraulic oil temperature sensor failure due to internal element degradation (common after 5,000+ operating hours)
• Corroded or damaged wiring harness connectors at the sensor plug location near the hydraulic tank
• Chafed or pinched sensor wiring where the harness routes along the swing bearing or undercarriage frame rails
• Water intrusion into connector pins causing intermittent short circuits
• ECM internal fault (less common, but possible on high-hour machines with voltage spike history)
• Poor ground connections at the sensor mounting point or main chassis ground

How to Troubleshoot and Fix Code H-27

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor (typically mounted on the hydraulic tank or return line manifold). Inspect the wiring harness for obvious damage, abrasion points where it contacts metal edges, or oil contamination that degrades insulation. Check the connector for corrosion, pushed-back pins, or moisture—common issues on used excavators stored outdoors.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure sensor resistance. At room temperature (68°F/20°C), resistance should read approximately 2,000-3,000 ohms (consult service manual for exact specifications). Test the wiring harness continuity from the sensor connector back to the ECM—readings should show less than 5 ohms resistance with no shorts to ground.

Step 3: Signal Voltage Check With the ignition on (engine off), backprobe the sensor connector to measure supply voltage—should read 5 volts ±0.25V. If voltage is absent or incorrect, trace the circuit back to the ECM, checking for damaged wires or failed ECM power supply circuits.

Step 4: Sensor Replacement and Verification If testing confirms sensor failure, replace with a genuine Komatsu temperature sensor or OEM-equivalent part. On used excavators, always apply dielectric grease to connector pins before reassembly to prevent future corrosion. Clear the fault code using Komatsu diagnostic software (KOMTRAX) or a compatible scan tool, then monitor for code recurrence during a 30-minute operational test cycle.

---

Disclaimer: This guide provides general diagnostic procedures. Always consult the official Komatsu PC55MR-2 service manual and consider professional technician assistance for complex electrical diagnostics or if you're unfamiliar with high-voltage systems.