What is Hyundai Excavator Fault Code 120-2?

Fault Code 120-2 indicates an Engine Coolant Temperature (ECT) Sensor Circuit Low Voltage condition in Hyundai excavators. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects a voltage signal from the coolant temperature sensor that falls below the manufacturer's specified threshold, typically below 0.2 volts.

The ECT sensor plays a critical role in engine management by monitoring coolant temperature and sending voltage signals to the ECM. This data directly affects fuel injection timing, idle speed control, and engine protection protocols. In Hyundai excavators, accurate coolant temperature readings are essential for preventing overheating, optimizing fuel efficiency, and ensuring proper hydraulic system performance during demanding operations.

Common Symptoms

When fault code 120-2 is active, operators may experience:

• Check Engine Light or warning lamp illuminated on the instrument cluster
• Hard starting or extended cranking time, especially in cold conditions
• Rough idle or engine running rich (black exhaust smoke)
• Engine derate mode activation, limiting power output to protect components
• Inaccurate temperature gauge readings showing extremely cold temperatures regardless of actual engine condition
• Cooling fan running continuously at high speed

Potential Causes

The most common technical causes for code 120-2 in used Hyundai excavators include:

• Damaged or shorted ECT sensor wiring harness, particularly at flex points near the engine block where vibration causes insulation wear
• Failed coolant temperature sensor with internal short circuit or contaminated sensing element
• Corroded or loose electrical connectors at the sensor or ECM terminals due to coolant leaks or moisture intrusion
• Broken or frayed wiring between the sensor and ECM, common in high-hour machines
• ECM internal fault or poor ground connection affecting sensor circuit reference voltage
• Previous repair attempts using incorrect sensor specifications or aftermarket components

How to Troubleshoot and Fix Code 120-2

Step 1: Visual Inspection Locate the ECT sensor (typically installed in the engine block or thermostat housing). Inspect the wiring harness for obvious damage, chafing, or coolant contamination. Check connector terminals for corrosion, bent pins, or moisture—critical on used excavators with extended outdoor exposure.

Step 2: Electrical Testing Using a digital multimeter, disconnect the ECT sensor connector and measure resistance across sensor terminals. At 68°F (20°C), resistance should typically read 2,000-3,000 ohms (consult Hyundai specifications). Check wiring continuity from sensor to ECM and verify less than 5 ohms resistance. Measure reference voltage from ECM—should read approximately 5 volts.

Step 3: Sensor and Harness Replacement If sensor resistance is out of specification or wiring shows continuity faults, replace the defective component. For used machines, always inspect harness routing for wear points and secure with proper clamps to prevent future failures. Clear codes using Hyundai diagnostic software (Hi-MATE or compatible scan tool) and perform a test cycle.

Disclaimer: This guide provides general diagnostic information. Always consult the official Hyundai service manual for your specific excavator model and consider professional technician assistance for complex electrical diagnostics.