What is Hyundai HX220 NL EPFC Fault Code 4155 (5746-3)?

Fault Code 4155 (5746-3) indicates a malfunction in the Engine Coolant Temperature (ECT) sensor circuit, specifically detecting an intermittent or out-of-range signal from the coolant temperature monitoring system. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) receives voltage readings from the ECT sensor that fall outside the manufacturer's specified parameters or display erratic behavior during engine operation.

For the Hyundai HX220 NL EPFC excavator, this code is critical because the ECM relies on accurate coolant temperature data to optimize fuel injection timing, control engine warm-up cycles, and prevent overheating conditions. An unreliable temperature signal can cause the engine to run inefficiently, enter protective derate mode, or fail to reach proper operating temperature, significantly impacting hydraulic performance and overall productivity on the jobsite.

Common Symptoms

• Check Engine Light or malfunction indicator lamp illuminated on the instrument cluster
• Engine running rich or experiencing rough idle, particularly during cold starts
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Reduced engine power or activation of limp mode to protect the engine from potential thermal damage
• Inaccurate temperature gauge readings that fluctuate erratically or remain stuck at one position

Potential Causes

The most common technical causes for Code 4155 in used Hyundai HX220 excavators include:

• ECT sensor degradation or internal resistance drift due to age and thermal cycling
• Corroded or loose electrical connectors at the sensor or ECM harness connection points
• Wiring harness damage from chafing against engine components, particularly near the thermostat housing where the sensor is typically mounted
• Contaminated coolant causing sensor element fouling or reduced thermal transfer
• Poor ground connection in the sensor circuit affecting voltage reference
• Intermittent ECM issues, though less common than sensor-related failures in older machines

How to Troubleshoot and Fix Code 4155 (5746-3)

Step 1: Visual Inspection and Connection Check Begin by inspecting the ECT sensor located near the thermostat housing on the engine block. Check for coolant leaks, corrosion on the connector pins, and physical damage to the wiring harness. On used excavators, pay special attention to harness routing where cables may rub against engine brackets or vibrate against metal surfaces. Clean all connector terminals with electrical contact cleaner and ensure tight connections.

Step 2: Electrical Testing with Diagnostic Tools Using a digital multimeter, measure the ECT sensor resistance by disconnecting the sensor and testing across its terminals. At room temperature (68°F/20°C), resistance should typically read between 2,000-3,000 ohms; consult your service manual for exact specifications. Check the supply voltage at the harness connector with the ignition on—it should read approximately 5 volts. Also verify ground continuity between the sensor ground wire and chassis ground (should be less than 1 ohm resistance).

Step 3: Sensor Replacement and System Verification If resistance values are out of specification or the wiring shows damage, replace the ECT sensor with an OEM or quality aftermarket part. For used machines, consider replacing the sensor pigtail connector simultaneously, as these often develop internal corrosion invisible from the outside. After installation with proper thread sealant, clear the fault code using Hyundai diagnostic software (Hi-Mate or compatible scan tool), and monitor live data during a complete warm-up cycle to confirm the temperature reading changes smoothly from cold to operating temperature (approximately 180-200°F/82-93°C).

Disclaimer: This guide provides general diagnostic information for experienced technicians. Always consult the official Hyundai service manual for your specific machine serial number and follow proper safety procedures. If you're uncomfortable performing electrical diagnostics, contact a certified Hyundai heavy equipment technician for professional assistance.