What is Komatsu PC120LC-6 Fault Code E104?

Fault Code E104 on the Komatsu PC120LC-6 indicates a malfunction in the hydraulic oil temperature sensor circuit. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormal voltage signal from the hydraulic oil temperature sensor, typically indicating an open circuit, short circuit, or sensor failure.

The hydraulic oil temperature sensor plays a critical role in protecting the PC120LC-6's hydraulic system. It monitors operating temperatures and signals the ECM to activate warning systems when temperatures exceed safe thresholds. On this model, proper temperature monitoring prevents component damage, maintains optimal viscosity for hydraulic fluid, and ensures efficient operation of the hydraulic pump and actuators.

Common Symptoms

When Code E104 is active, operators typically experience:

• Hydraulic oil temperature warning light illuminated on the instrument cluster
• Erratic or absent temperature gauge readings showing either maximum temperature or no reading at all
• Reduced hydraulic performance as the system may enter protective derate mode
• Audible warning alarms that activate intermittently or continuously
• ECM logging the fault even when hydraulic oil feels normal to touch

Potential Causes

The most common technical causes for E104 on used PC120LC-6 excavators include:

• Sensor failure due to age-related deterioration or exposure to extreme temperatures
• Wiring harness damage at common rub points near the hydraulic tank and pump assembly
• Corroded or loose connector pins at the sensor plug, particularly on machines operated in wet conditions
• Open or short circuit in the sensor wiring between the sensor and ECM
• ECM internal fault (rare) affecting the temperature monitoring circuit
• Contaminated hydraulic fluid causing sensor element fouling or coating

How to Troubleshoot and Fix Code E104

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor (typically mounted on the hydraulic tank). Inspect the wiring harness for obvious damage, chafing, or breaks, especially where it routes near moving components. Check the connector for corrosion, bent pins, or oil contamination—common issues on older excavators.

Step 2: Electrical Testing Disconnect the sensor and use a digital multimeter to measure resistance across the sensor terminals. At room temperature (20°C), resistance should typically read 2,000-3,000 ohms (consult your service manual for exact specifications). Also check for continuity in the wiring from the sensor connector back to the ECM using a wiring diagram.

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 from the ECM. No voltage indicates a wiring fault or ECM issue.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Komatsu sensor or quality aftermarket equivalent. Before installation, clean the mounting boss and apply appropriate thread sealant. Clear the fault code using Komatsu diagnostic software (KOMTRAX or compatible scanner) and verify the repair through a temperature monitoring test cycle.

Disclaimer: This guide provides general troubleshooting information. Always consult your Komatsu PC120LC-6 service manual for model-specific procedures and specifications. For complex electrical diagnostics or if you're unfamiliar with high-voltage systems, contact a certified Komatsu technician or qualified heavy equipment mechanic.