What is Komatsu Fault Code E0391?

Komatsu fault code E0391 indicates a malfunction in the hydraulic oil temperature sensor circuit, specifically detecting an abnormal voltage signal or out-of-range temperature reading from the hydraulic system's temperature monitoring circuit. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) receives voltage values outside the expected parameters from the hydraulic oil temperature sensor, typically installed in the main hydraulic tank or return line.

This code is critical for Komatsu excavators because hydraulic oil temperature directly affects system efficiency, component longevity, and operational safety. When the ECM cannot accurately monitor hydraulic temperature, the machine may enter a protective derate mode to prevent catastrophic damage to pumps, motors, and seals. In used excavators, this sensor circuit is particularly vulnerable to degradation from years of heat cycling, vibration, and exposure to hydraulic fluid contamination.

Common Symptoms

When fault code E0391 is active, operators typically experience:

• Warning light illumination on the instrument panel, often accompanied by an audible alarm
• Reduced hydraulic power or sluggish machine response as the ECM limits performance
• Inaccurate or frozen hydraulic temperature gauge readings on the monitor display
• Engine derate or automatic shutdown if the system assumes overheating conditions
• Intermittent code activation during cold starts or after extended operation periods

Potential Causes

The most common technical causes for E0391 in used Komatsu excavators include:

• Faulty hydraulic oil temperature sensor due to internal element failure or contamination exposure
• Damaged or corroded wiring harness connections at the sensor plug, especially where harnesses route near hot engine components
• Open or short circuits in the sensor wiring caused by chafing against frame members or hydraulic lines
• Corroded connector pins at the ECM or sensor junction points from moisture intrusion
• Failed ECM internal circuitry (rare, but possible in high-hour machines)
• Contaminated hydraulic oil causing sensor element coating and false readings

How to Troubleshoot and Fix Code E0391

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor (consult your service manual for exact location—typically in the hydraulic tank). Inspect the sensor connector and wiring harness for obvious damage, corrosion, oil contamination, or worn insulation. On used excavators, pay special attention to harness routing where cables may rub against hydraulic tubes or structural components.

Step 2: Electrical Testing Disconnect the sensor and use a digital multimeter to check sensor resistance. At ambient temperature (68°F/20°C), resistance should typically range between 2,000-3,000 ohms (verify exact specifications in your Komatsu service manual). Check for open circuits or short to ground in the harness by measuring continuity between the sensor connector pins and the ECM connector with the harness disconnected at both ends.

Step 3: Voltage Supply Verification With the ignition on and sensor disconnected, measure the reference voltage at the sensor harness connector. You should see approximately 5 volts on the signal wire. If voltage is absent or incorrect, trace the circuit back toward the ECM, checking for damaged wires or poor ground connections.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Komatsu OEM sensor or quality aftermarket equivalent. Before installation, clean the sensor port thoroughly and apply dielectric grease to connector terminals to prevent future corrosion. After replacement, clear the fault code using Komatsu diagnostic software (KOMTRAX or compatible scan tool) and verify proper operation under normal temperature conditions.

Used Equipment Consideration: On high-hour machines, always inspect hydraulic oil condition—contaminated oil can prematurely damage new sensors. Consider replacing corroded connectors and securing harnesses away from heat sources to prevent recurrence.

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Disclaimer: This guide provides general troubleshooting information. Always consult your Komatsu service manual and consider professional diagnostic assistance for complex electrical issues or if you lack proper diagnostic tools.