What is Komatsu PC120LC-6 Fault Code H-20?

Fault Code H-20 on the Komatsu PC120LC-6 indicates a hydraulic oil temperature sensor circuit malfunction. This code triggers when the Engine Control Module (ECM) detects abnormal voltage readings from the hydraulic oil temperature sensor, typically outside the normal operating range of 0.5-4.5 volts.

The hydraulic oil temperature sensor monitors the temperature of hydraulic fluid circulating through the excavator's system. This data is critical for the ECM to prevent overheating damage to hydraulic pumps, motors, and seals. When this sensor fails or sends erratic signals, the machine cannot accurately protect itself from thermal damage, potentially leading to catastrophic hydraulic system failure. On used PC120LC-6 excavators, this fault is particularly concerning as aging hydraulic components are more vulnerable to heat-related deterioration.

Common Symptoms

When Code H-20 is active, operators typically experience:

• Hydraulic oil temperature gauge reading incorrectly or showing no reading at all
• Warning light illuminated on the instrument panel, possibly accompanied by audible alarm
• Machine may enter derate mode, limiting hydraulic flow or engine RPM to prevent potential damage
• Inaccurate temperature readings making it impossible to monitor actual hydraulic system health
• Intermittent code appearance, especially during warm-up or under heavy load conditions

Potential Causes

The most common technical causes for H-20 on used PC120LC-6 excavators include:

• Temperature sensor failure due to internal resistance drift or element breakdown (common after 5,000+ operating hours)
• Wiring harness damage at known rub points near the hydraulic tank and along the main boom cylinder routing
• Connector corrosion at the sensor plug or ECM connection points, especially on machines operated in wet or coastal environments
• ECM internal circuit fault affecting the sensor input channel (rare but possible on high-hour machines)
• Broken or chafed wires in the sensor circuit, particularly where harnesses pass through cab mounting points
• Poor ground connection affecting sensor reference voltage

How to Troubleshoot and Fix Code H-20

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor, typically mounted on the hydraulic tank or return line manifold. Inspect the sensor connector for corrosion, pin damage, or oil contamination. On used excavators, carefully trace the wiring harness back to the ECM, checking known wear points where cables contact the frame or hydraulic lines.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance at various temperatures. A functional sensor should show approximately 2,000-3,000 ohms at 20°C, decreasing as temperature rises. Check harness continuity from sensor to ECM (should read less than 5 ohms) and verify no shorts to ground (should read infinite resistance).

Step 3: Voltage Circuit Check With the ignition on and sensor disconnected, measure supply voltage at the harness connector. You should see approximately 5 volts reference from the ECM. If voltage is absent or incorrect, suspect ECM issues or harness breaks.

Step 4: Component Replacement If sensor resistance is out of specification or the harness shows damage, replace the faulty component. For used machines, always replace connector seals and apply dielectric grease to prevent future corrosion. Clear the code using Komatsu diagnostic software (KOMTRAX or compatible scan tool) and verify repair by operating the machine through a full temperature cycle.

---

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu service manual for your specific serial number and seek professional assistance for complex electrical diagnostics or if you lack proper diagnostic tools.