What is Komatsu Fault Code E601?

Komatsu fault code E601 indicates a malfunction in the hydraulic oil temperature sensor circuit, specifically detecting an abnormally high voltage signal or open circuit condition in the sensor wiring. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) or monitor panel receives a voltage reading outside the acceptable range from the hydraulic oil temperature sensor, typically above 4.8-4.9 volts.

This code is critical for Komatsu excavators because hydraulic oil temperature directly affects system efficiency, component longevity, and safe operation. The ECM relies on accurate temperature data to protect the hydraulic system from overheating, adjust pump output, and activate cooling fans. When E601 appears, the system may enter a default mode using estimated temperature values, potentially compromising machine protection and performance.

Common Symptoms

• Warning light illumination on the instrument panel, often accompanied by a hydraulic temperature icon or general malfunction indicator
• Reduced hydraulic performance or sluggish operation as the system enters protective derate mode
• Inaccurate or frozen temperature gauge readings, typically showing maximum temperature or no reading at all
• Continuous cooling fan operation regardless of actual hydraulic oil temperature
• Machine may enter limp mode on some Komatsu models, limiting engine RPM or hydraulic flow

Potential Causes

The most common causes of fault code E601 on used Komatsu excavators include:

• Open circuit or broken wiring in the sensor harness, particularly at flex points near the hydraulic tank where cables experience repeated bending
• Corroded or loose connector pins at the sensor plug or ECM connection points (common in machines with high operating hours)
• Failed hydraulic oil temperature sensor, where internal resistance becomes infinite or the sensing element separates
• Damaged sensor connector seals allowing moisture intrusion and causing high resistance or intermittent connections
• Chafed wiring harness rubbing against frame members or hydraulic lines, creating intermittent opens or shorts to voltage
• ECM internal circuit failure (rare, but possible in older machines with electrical system stress)

How to Troubleshoot and Fix Code E601

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, bent pins, oil contamination, or moisture. Check the entire wiring harness from sensor to ECM for visible damage, particularly at harness routing points where friction occurs. On used excavators, pay special attention to areas where protective loom has deteriorated.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure sensor resistance. At room temperature (20°C/68°F), resistance should typically read 2,000-3,000 ohms (consult your specific Komatsu service manual for exact specifications). Check for continuity in the wiring harness between the sensor connector and ECM pins. Measure voltage supply at the sensor connector with ignition on—you should see approximately 5 volts reference voltage from the ECM.

Step 3: Circuit and Component Verification With the sensor disconnected, check for short circuits to ground or voltage on the signal wire. If wiring tests good, install a new OEM or quality aftermarket sensor and clear the code using Komatsu diagnostic software (KOMTRAX or compatible scan tool). For used machines, always apply dielectric grease to connector pins before reassembly to prevent future corrosion.

Step 4: Final Validation After repairs, operate the machine through a complete warm-up cycle while monitoring live data with diagnostic software. Verify the temperature reading changes appropriately (typically 40-80°C/104-176°F during normal operation) and confirm code E601 does not return.

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Disclaimer: This guide provides general troubleshooting information for Komatsu fault code E601. Always consult your machine-specific service manual and consider professional diagnostic assistance for complex electrical issues. Improper repairs may cause additional damage or safety hazards.