What is Hyundai Excavator Fault Code 135-2?

Fault Code 135-2 indicates an Engine Coolant Temperature (ECT) Sensor Circuit High Voltage condition in Hyundai excavators. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormally high voltage signal from the coolant temperature sensor circuit, typically exceeding 4.8-4.9 volts for more than 0.5 seconds.

The ECT sensor plays a critical role in engine management by monitoring coolant temperature and sending voltage signals to the ECM. This data directly influences fuel injection timing, idle speed control, and engine protection protocols. When the ECM receives a high voltage reading, it interprets this as an extremely cold temperature (often below -40°C), which contradicts normal operating conditions. In Hyundai excavators, this fault can trigger protective derate modes that limit machine performance to prevent potential engine damage.

Common Symptoms

• Check Engine Light or warning lamp illuminated on the instrument cluster
• Hard starting conditions, especially when the engine is warm, due to incorrect fuel mapping
• Rough idle or hunting idle speed as the ECM compensates for false temperature readings
• Black smoke from the exhaust caused by overly rich fuel mixture
• Engine derate mode activation, limiting RPM and hydraulic performance to protect the engine
• Poor fuel economy resulting from incorrect combustion parameters

Potential Causes

The most common technical causes for Code 135-2 in used Hyundai excavators include:

• Open circuit in the ECT sensor wiring harness, often caused by wire breakage or connector damage
• Failed ECT sensor with internal circuit failure creating high resistance
• Corroded or damaged connectors at the sensor or ECM—extremely common in used machines exposed to moisture
• Harness chafing at known rub points near the thermostat housing or along the valve cover
• Poor ground connection at the sensor mounting point due to corrosion or loose threads
• ECM internal fault (rare) affecting the 5-volt reference circuit
• Aftermarket sensor incompatibility if previous repairs used non-OEM parts

How to Troubleshoot and Fix Code 135-2

Step 1: Visual Inspection Begin with a thorough physical inspection of the ECT sensor located near the thermostat housing. Check for damaged wiring, corroded connector pins, or coolant contamination at the sensor base. On used excavators, inspect harness routing for wear points where cables contact the engine block or frame—this is a frequent failure point after 5,000+ operating hours.

Step 2: Electrical Testing Disconnect the ECT sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At 20°C (68°F), resistance should typically read 2,000-3,000 ohms (consult your specific service manual for exact specifications). If resistance is infinite or extremely high (>100k ohms), the sensor has failed internally and requires replacement.

Step 3: Circuit Voltage Check With the sensor disconnected and ignition on (engine off), measure voltage at the harness connector. You should see approximately 5 volts on the signal wire (reference voltage from ECM). If voltage is absent or incorrect, trace the wiring back to the ECM connector checking for breaks, shorts to ground, or damaged pins.

Step 4: Harness Continuity Test Test continuity between the sensor connector and ECM pins using wiring diagrams specific to your Hyundai model. Pay special attention to the ground circuit—clean the sensor mounting threads and apply dielectric grease to prevent future corrosion. For used machines, consider replacing the entire sensor harness if multiple wear points are evident.

Step 5: Component Replacement Install a genuine Hyundai ECT sensor (OEM part recommended for accurate calibration). After installation, clear fault codes using Hi-SCANPro diagnostic software or equivalent Hyundai-compatible scan tool. Perform a cold start test cycle to verify proper operation and confirm the code does not return.

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Disclaimer: This guide provides general troubleshooting information for Hyundai excavator Fault Code 135-2. Always consult your machine's specific service manual and consider professional diagnostic assistance for complex electrical issues. Improper repairs may cause further damage or safety hazards.