Komatsu PC850-8R Fault Code H-19: Complete Diagnostic Guide

What is Komatsu PC850-8R Fault Code H-19?

Fault Code H-19 indicates a hydraulic oil temperature sensor circuit malfunction in the Komatsu PC850-8R excavator. This code is triggered when the Electronic Control Module (ECM) detects an abnormal voltage signal from the hydraulic oil temperature sensor, typically reading values outside the normal operating range of 0.5-4.5 volts.

The hydraulic oil temperature sensor is critical for the PC850-8R's performance management system. It monitors the temperature of hydraulic fluid flowing through the main circuit, allowing the ECM to adjust pump output and implement protective measures when temperatures exceed safe operating limits. When this sensor fails or provides erratic readings, the machine cannot properly regulate hydraulic system performance, potentially leading to component damage or reduced operational efficiency.

Common Symptoms

When Code H-19 is active, operators typically experience:

• Warning light illumination on the monitor panel, often accompanied by a hydraulic temperature warning icon
• Reduced hydraulic performance or derated machine functions as the ECM enters protective mode
• Inaccurate temperature readings displayed on the cab monitor, showing extremely high or low values (often -40°C or 150°C+)
• Intermittent loss of auxiliary hydraulic functions during operation
• Error messages on the multi-function display indicating hydraulic system faults

Potential Causes

The most common technical causes for H-19 in used PC850-8R excavators include:

• Damaged or corroded sensor connector at the hydraulic tank mounting location, exposed to moisture and debris
• Failed hydraulic oil temperature sensor due to thermal cycling and age-related degradation
• Wiring harness damage along the routing path from the sensor to the ECM, particularly near the swing bearing where abrasion occurs
• Poor ground connections at the sensor mounting point or ECM ground terminals
• ECM internal circuit failure (rare, but possible in high-hour machines)
• Contaminated hydraulic oil causing sensor element fouling

How to Troubleshoot and Fix Code H-19

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor on the main hydraulic tank. Inspect the sensor connector for corrosion, oil contamination, or damaged pins. On used excavators, this connector frequently suffers from moisture intrusion. Clean contacts with electrical contact cleaner and check for proper seating.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure resistance across the sensor terminals. At 20°C (68°F), resistance should read approximately 2,000-3,000 ohms (consult your service manual for exact specifications). Check wiring harness continuity from the sensor connector back to the ECM, ensuring no opens or shorts to ground.

Step 3: Voltage Signal Verification With the sensor connected and ignition on (engine off), backprobe the signal wire at the sensor connector. You should read approximately 5 volts reference voltage from the ECM. Start the engine and monitor voltage as the system warms—it should gradually decrease as temperature rises (typically 3.5V cold to 1.5V hot).

Step 4: Harness and Mounting Inspection For used machines, carefully trace the sensor harness along its entire route, paying special attention to areas where it passes near moving components or sharp edges. Check the sensor mounting threads for corrosion and ensure proper ground contact with the tank body.

Step 5: Component Replacement If testing confirms sensor failure, replace with a genuine Komatsu sensor (Part #7861-93-3320 or equivalent). After replacement, clear codes using Komatsu diagnostic software (KOMTRAX) and verify proper operation through a complete heat cycle.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu PC850-8R service manual and consider professional assistance from certified Komatsu technicians for complex repairs. Improper diagnosis may lead to equipment damage or safety hazards.