What is Komatsu Fault Code E0155?

Komatsu fault code E0155 indicates a malfunction in the Hydraulic Oil Temperature Sensor Circuit, specifically detecting an abnormal voltage signal or open/short circuit condition in the sensor wiring or the sensor itself. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) receives sensor readings outside the expected voltage range, typically below 0.2V or above 4.8V on a 5V reference circuit.

This code is critical for Komatsu excavators because hydraulic oil temperature directly affects system viscosity, component wear rates, and overall machine efficiency. The ECM relies on accurate temperature data to protect hydraulic pumps, motors, and cylinders from thermal damage. When E0155 activates, the system may enter a protective mode that limits machine performance to prevent catastrophic hydraulic system failure.

Common Symptoms

• Hydraulic oil temperature warning lamp illuminated on the instrument cluster, often accompanied by an audible alarm
• Reduced hydraulic response or sluggish operation as the ECM may default to conservative performance parameters
• Intermittent loss of temperature readings on the monitor display, showing dashes or error symbols instead of numerical values
• Engine derate mode activation in some Komatsu models, limiting RPM to protect the hydraulic system
• Stored fault code in ECM memory even after the warning light extinguishes, requiring diagnostic scanner retrieval

Potential Causes

• Damaged or corroded sensor connector at the hydraulic tank, a common issue in used excavators exposed to moisture and debris
• Broken or chafed wiring harness between the temperature sensor and ECM, particularly at routing points near the swing motor or tank mounting brackets
• Failed hydraulic oil temperature sensor due to age-related internal component degradation or thermal stress
• Faulty ECM voltage reference circuit providing incorrect 5V supply to the sensor
• Corroded ground connections in the sensor circuit causing intermittent high-resistance faults
• Previous repair attempts using incorrect sensor specifications or improper wiring modifications

How to Troubleshoot and Fix Code E0155

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor, typically mounted on the hydraulic tank or return line filter housing. Inspect the connector for corrosion, bent pins, moisture intrusion, or oil contamination. On used excavators, check for harness wear at friction points where wiring contacts frame members or hydraulic lines.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At 20°C (68°F), most Komatsu temperature sensors should read approximately 2,000-3,000 ohms (consult your specific model's service manual). Check continuity from the sensor connector back to the ECM harness connector, verifying no opens or shorts to ground in the wiring.

Step 3: Voltage Reference Check With the ignition on and sensor disconnected, measure voltage at the sensor harness connector. You should see approximately 5V on the signal wire. If voltage is absent or incorrect, trace the reference voltage circuit back to the ECM, checking for damaged wiring or poor ECM connections.

Step 4: Sensor Replacement and Verification If wiring tests pass but the sensor resistance is out of specification, replace the hydraulic oil temperature sensor with a genuine Komatsu part. After installation, clear the fault code using Komatsu diagnostic software (KOMTRAX or equivalent) and operate the machine through a complete heating cycle to verify the repair.

For used excavators, always apply dielectric grease to connector pins during reassembly to prevent future corrosion, and secure harnesses away from moving components with proper clips.

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Disclaimer: This guide provides general troubleshooting information. Always consult your Komatsu service manual for model-specific procedures and safety precautions. If you lack diagnostic equipment or experience with hydraulic systems, seek assistance from a certified Komatsu technician.