What is Komatsu Fault Code E0377?

Komatsu Fault Code E0377 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. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) receives sensor readings outside the normal operating range, typically between 0.5-4.5 volts.

The hydraulic oil temperature sensor plays a critical role in monitoring fluid viscosity and protecting the excavator's hydraulic system from overheating damage. When E0377 activates, the ECM may default to a predetermined temperature value, potentially causing inefficient operation or protective derate modes. For used Komatsu excavators, this code often appears due to sensor degradation, corroded connectors, or damaged wiring harnesses—common issues in machines with 5,000+ operating hours.

Common Symptoms

• Warning light illuminated on the instrument cluster, often accompanied by a hydraulic system icon
• Reduced hydraulic performance or slower cycle times as the ECM enters protective mode
• Inaccurate temperature readings displayed on the monitor, showing extremely high or low values (often -40°C or 300°C+)
• Intermittent code activation during machine operation, particularly when traveling over rough terrain
• Fan operation irregularities, with cooling fans running continuously or not engaging at appropriate temperatures

Potential Causes

The most common technical reasons for E0377 on used Komatsu excavators include:

• Failed hydraulic oil temperature sensor due to internal resistance drift or element failure (common after 8,000+ hours)
• Corroded or loose connector pins at the sensor harness connection point near the hydraulic tank
• Damaged wiring harness caused by rubbing against frame components, particularly along the chassis routing path
• Shorted sensor wiring from pinched cables during previous maintenance or track frame flex points
• ECM internal fault (rare, but possible in high-hour machines with moisture exposure)
• Contaminated hydraulic oil causing sensor element coating and false readings

How to Troubleshoot and Fix Code E0377

Step 1: Visual Inspection and Connector Check Begin by locating the hydraulic oil temperature sensor, typically mounted on the hydraulic tank or return line manifold. Inspect the sensor connector for corrosion, oil contamination, or pushed-back pins. On used excavators, carefully examine the wiring harness routing for chafe marks, especially where it passes near moving components or sharp edges. Clean connectors with electrical contact cleaner and apply dielectric grease.

Step 2: Sensor Resistance Testing Disconnect the sensor and measure resistance across the sensor terminals using a digital multimeter. At room temperature (20°C/68°F), resistance should typically read between 2,000-3,000 ohms (consult your specific model's service manual). Compare readings at different temperatures—resistance should decrease smoothly as temperature increases. Erratic or out-of-range readings indicate sensor replacement is necessary.

Step 3: Circuit Voltage and Continuity Testing With the sensor disconnected and ignition on, measure supply voltage at the harness connector (should read approximately 5 volts). Check for continuity between the sensor ground and chassis ground (should read less than 1 ohm). Test the signal wire for shorts to ground or power. If wiring tests fail, trace the harness back to the ECM connector, checking for damaged insulation or corroded pins at connection points.

Step 4: Harness Repair and Component Replacement For used machines, harness damage is frequently the root cause. Repair any damaged wiring using proper gauge wire and heat-shrink connections—avoid simple electrical tape in hydraulic compartment environments. If the sensor tests faulty, replace it with a genuine Komatsu part or OEM-equivalent temperature sensor, ensuring proper thread sealant application. After repairs, clear the fault code using Komatsu diagnostic software (KTTS) or a compatible scanner, then operate the machine through a full heat cycle to verify the repair.

Disclaimer: This guide provides general troubleshooting information for Komatsu E0377 fault codes. Hydraulic and electrical system diagnosis can be complex and may require specialized tools and training. Always consult your machine's specific service manual and consider professional diagnostic assistance for persistent issues or if you're unfamiliar with high-voltage electrical systems.