What is Komatsu PC120LC-6 Fault Code H-19?

Fault Code H-19 on the Komatsu PC120LC-6 indicates a hydraulic oil temperature sensor circuit malfunction. This code triggers when the Electronic Control Module (ECM) detects abnormal voltage readings from the hydraulic oil temperature sensor, which monitors the operating temperature of the hydraulic system fluid.

The hydraulic oil temperature sensor plays a critical role in protecting the PC120LC-6's hydraulic system from overheating damage. When functioning properly, this sensor sends voltage signals to the ECM, allowing the machine to activate warning systems or reduce performance before catastrophic component failure occurs. On used excavators, this sensor circuit is particularly vulnerable to degradation from years of vibration, moisture intrusion, and thermal cycling.

Common Symptoms

• Hydraulic oil temperature warning light illuminated on the instrument panel, even when the oil feels cool to touch
• Intermittent or continuous alarm buzzer activation during operation
• Reduced hydraulic performance as the ECM may enter a protective derate mode
• Erratic temperature gauge readings that fluctuate abnormally or show maximum temperature immediately at startup
• ECM storing fault history that prevents the code from clearing even after cooling periods

Potential Causes

The H-19 code typically results from several age-related failures common in used Komatsu excavators:

• Sensor internal resistance drift due to thermal stress over thousands of operating hours
• Corroded or loose connector pins at the sensor harness connection point near the hydraulic tank
• Damaged wiring harness where cables run along the boom cylinder area—a known friction point on PC120LC-6 models
• Contaminated sensor threads with metallic debris or hardened sealant preventing proper thermal contact
• ECM voltage supply issues affecting the 5-volt reference circuit to the sensor
• Faulty sensor ground connection caused by corrosion on chassis mounting points

How to Troubleshoot and Fix Code H-19

Step 1: Visual Inspection Locate the hydraulic oil temperature sensor on the hydraulic tank (typically on the return line side). Inspect the wiring harness for physical damage, oil contamination, or wear marks where cables contact metal surfaces. On used excavators, check for evidence of previous repairs or wire splicing.

Step 2: Connector and Resistance Testing Disconnect the sensor connector and inspect pins for corrosion or bent contacts. Using a digital multimeter, measure sensor resistance across the sensor terminals. At 20°C (68°F), resistance should typically read 2.0-2.5 kΩ (consult your service manual for exact specifications). Compare readings with a known-good sensor if available.

Step 3: Circuit Voltage Check With the connector still disconnected and ignition ON, measure voltage supply at the harness connector. You should see approximately 5 volts DC on the signal wire. If voltage is absent or incorrect, trace the wiring harness back to the ECM, checking for breaks or shorts to ground.

Step 4: Sensor Replacement and Verification If testing confirms sensor failure, remove the old unit and inspect the mounting threads for debris buildup—common in older machines. Apply appropriate thread sealant (not Teflon tape, which can contaminate hydraulics) and install the new sensor to manufacturer torque specifications. Clear codes using Komatsu diagnostic software and verify proper operation through a heat cycle.

Disclaimer: This guide provides general troubleshooting information. Hydraulic system repairs involve pressurized components and electrical diagnostics require proper training. Always consult the official Komatsu service manual for your specific serial number and consider professional assistance for complex electrical issues.