Komatsu PC55MR-2 Fault Code H-20: Complete Diagnostic Guide

What is Komatsu PC55MR-2 Fault Code H-20?

Fault Code H-20 on the Komatsu PC55MR-2 indicates a hydraulic oil temperature sensor circuit malfunction or abnormal temperature reading in the hydraulic system. This diagnostic trouble code specifically monitors the hydraulic oil temperature sensor circuit, which reports fluid temperature data to the machine's Electronic Control Unit (ECU).

This code is critical for the PC55MR-2's performance because hydraulic oil temperature directly affects system efficiency, component wear, and operational safety. When the ECU cannot accurately monitor hydraulic temperatures, it may impose operational restrictions to prevent catastrophic damage to pumps, motors, and cylinders. The hydraulic system is the lifeblood of excavator operation, making this sensor essential for protecting your investment.

Common Symptoms

When Code H-20 is active on your Komatsu PC55MR-2, operators typically experience:

• Warning light illumination on the instrument panel (hydraulic temperature icon or general warning lamp)
• Reduced hydraulic performance including slower cycle times and decreased digging force
• Audible warning alarm during startup or continuous operation
• Machine may enter derated mode (limited RPM or hydraulic flow) as a protective measure
• Intermittent code activation during cold starts or after extended operation periods

Potential Causes

The H-20 code on used PC55MR-2 excavators commonly stems from these technical issues:

• Hydraulic oil temperature sensor failure due to internal element degradation (common after 5,000+ operating hours)
• Damaged or corroded wiring harness connections at the sensor plug or ECU terminals
• Harness chafing against the swing motor housing or hydraulic tank mounting brackets (known wear point on this model)
• Contaminated hydraulic oil causing actual overheating that triggers legitimate sensor readings
• ECU voltage supply issues providing incorrect reference voltage to the sensor circuit
• Defective ECU with internal circuit board damage (rare, but occurs in machines exposed to moisture intrusion)

How to Troubleshoot and Fix Code H-20

Step 1: Visual Inspection Begin by inspecting the hydraulic oil temperature sensor location near the hydraulic tank return line. Check for physical damage, oil contamination on connectors, and corrosion on terminal pins. On used excavators, examine the entire harness routing from sensor to ECU for rubbing, cracking, or rodent damage.

Step 2: Electrical Testing Using a digital multimeter, perform these checks with ignition ON, engine OFF:

• Measure sensor resistance (typically 200-5,000 ohms depending on temperature—consult service manual for specification chart)
• Check supply voltage at the sensor connector (should read approximately 5 volts)
• Test harness continuity between sensor connector and ECU pins
• Inspect for short circuits to ground in the signal wire

Step 3: Advanced Diagnostics Connect Komatsu diagnostic software (KOMTRAX or compatible scan tool) to read live data. Compare actual hydraulic oil temperature against sensor readings. If readings are erratic or frozen, suspect sensor failure. If actual temperature exceeds 80°C (176°F) frequently, address cooling system issues before replacing sensors.

Step 4: Component Replacement Protocol For used machinery, replace corroded connectors before condemning sensors. Clean all terminal contacts with electrical contact cleaner. If sensor replacement is necessary, always use genuine Komatsu parts (PN 7861-93-3320 or equivalent) and apply dielectric grease to connections to prevent future corrosion.

---

Disclaimer: This guide provides general troubleshooting information for the Komatsu PC55MR-2 fault code H-20. Hydraulic and electrical system diagnostics can be complex and potentially dangerous. Always consult your machine's official service manual, and consider engaging a certified Komatsu technician for proper diagnosis and repair, especially on used equipment with unknown maintenance history.