Komatsu PC120-6 Fault Code H-20: Complete Diagnostic Guide

What is Komatsu PC120-6 Fault Code H-20?

Fault Code H-20 indicates a malfunction in the hydraulic oil temperature sensor circuit or abnormally high hydraulic oil temperature. This code is triggered when the Electronic Control Module (ECM) detects either a sensor voltage outside the normal operating range (0.5-4.5V) or actual oil temperatures exceeding 110°C (230°F).

This code is critical for the PC120-6 because excessive hydraulic oil temperature degrades system efficiency, accelerates component wear, and can cause catastrophic failures in the main hydraulic pump, control valves, and sealing systems. The hydraulic system operates optimally between 50-80°C, and H-20 activates protective measures to prevent permanent damage.

Common Symptoms

• Flashing warning lamp on the instrument panel accompanied by an audible buzzer
• Reduced hydraulic response or sluggish boom, arm, and swing movements as the ECM enters protective derate mode
• Visible smoke or burning smell from the hydraulic tank area in severe overheating cases
• Erratic temperature gauge readings fluctuating between normal and maximum positions
• Automatic machine shutdown if temperatures exceed critical thresholds (above 120°C)

Potential Causes

• Faulty hydraulic oil temperature sensor – common failure point on machines with 5,000+ hours due to thermal cycling and contamination
• Damaged wiring harness at sensor connector locations, especially near the hydraulic tank where vibration and heat cause insulation breakdown
• Clogged hydraulic oil cooler resulting from debris accumulation in the cooling fins or internal passages
• Low hydraulic oil level creating insufficient fluid for proper heat dissipation
• Worn hydraulic pump generating excessive heat due to internal leakage and reduced volumetric efficiency
• Corroded or loose electrical connectors at the sensor or ECM interface, typical in used excavators exposed to moisture

How to Troubleshoot and Fix Code H-20

Step 1: Visual Inspection Check the hydraulic oil level at the sight glass with the engine off. Inspect the oil cooler fins for blockage from mud, debris, or damaged fins. Examine the wiring harness from the temperature sensor (located on the hydraulic tank) to the ECM for chafing, cuts, or connector corrosion.

Step 2: Sensor Circuit Testing Using a digital multimeter, disconnect the temperature sensor connector and measure resistance across sensor terminals. At 20°C (68°F), resistance should be approximately 2.4-2.8 kΩ. Check for voltage supply at the harness connector: should read approximately 5V with ignition on. Resistance outside specification indicates sensor replacement is required.

Step 3: Physical Temperature Verification Use an infrared thermometer to measure actual hydraulic oil temperature at the tank. Compare with gauge/ECM readings. If actual temperature is normal (below 90°C) but code persists, replace the temperature sensor (part number 7861-93-3320). If temperature is genuinely high, clean or replace the hydraulic oil cooler and verify proper fan operation.

Step 4: Used Equipment-Specific Checks Inspect connector pins for green corrosion buildup—common on older PC120-6 models. Apply dielectric grease after cleaning. Check for harness rub points near the swing bearing and along the upper frame. Replace deteriorated hydraulic oil if contaminated, as particle buildup reduces cooling efficiency.

Disclaimer: This guide provides general troubleshooting procedures. Always consult the official Komatsu service manual and qualified hydraulic technicians for complex repairs, especially on pressurized hydraulic systems.