What is Hyundai HX220 NL EPFC Fault Code 135 (100-3)?

Fault Code 135 (100-3) on the Hyundai HX220 NL EPFC indicates an Engine Coolant Temperature (ECT) sensor circuit malfunction or abnormal voltage signal detected by the Engine Control Module (ECM). This code specifically points to the ECT sensor providing readings outside the acceptable voltage range, typically caused by sensor failure, wiring issues, or poor electrical connections within the engine monitoring system.

The ECT sensor plays a critical role in the HX220's EPFC (Electronic Pump Flow Control) system by providing real-time coolant temperature data to the ECM. This information directly affects fuel injection timing, engine warm-up cycles, cooling fan activation, and hydraulic system performance. When this sensor fails or sends erratic signals, the excavator's performance management system cannot properly regulate engine operations, potentially leading to inefficient fuel consumption, overheating risks, or reduced hydraulic power output.

Common Symptoms

• Check Engine Light or warning indicator illuminated on the instrument panel
• Engine running in derate mode (reduced power output) as a protective measure
• Cooling fan running continuously at high speed, regardless of actual engine temperature
• Difficulty starting the engine when cold, or extended cranking times during warm starts
• Erratic temperature gauge readings or gauge stuck at minimum/maximum positions

Potential Causes

The most common technical reasons for Code 135 (100-3) on used HX220 excavators include:

• Failed ECT sensor due to internal element deterioration (common after 5,000+ operating hours)
• Corroded or loose connector pins at the ECT sensor harness connection point
• Damaged sensor wiring harness, particularly where cables route near the exhaust manifold or vibration-prone areas
• Contaminated coolant causing sensor element coating or degraded sensor accuracy
• ECM internal fault (less common) affecting the sensor signal processing circuit
• Poor ground connections at the engine block or sensor mounting location

How to Troubleshoot and Fix Code 135 (100-3)

Step 1: Visual Inspection Begin by locating the ECT sensor (typically mounted on the engine cylinder head or thermostat housing). Inspect the sensor connector for corrosion, moisture intrusion, or damaged pins. On used excavators, check for harness chafing where wires contact metal surfaces or route near hot engine components.

Step 2: Electrical Testing Disconnect the ECT sensor connector and use a digital multimeter to measure sensor resistance. At 20°C (68°F), resistance should typically read 2,000-3,000 ohms (consult your service manual for exact specifications). Check harness continuity from the sensor connector back to the ECM, and verify ground circuit integrity (should read less than 1 ohm resistance).

Step 3: Signal Voltage Check With the ignition on (engine off), measure reference voltage at the sensor connector from the ECM—should read approximately 5 volts. If voltage is absent or incorrect, trace the signal wire for breaks or shorts to ground.

Step 4: Sensor Replacement and Verification If testing confirms sensor failure, replace with a genuine Hyundai ECT sensor or OEM-equivalent part. Apply dielectric grease to connector pins before reconnection. Clear the fault code using Hyundai diagnostic software or a compatible J1939 scan tool, then monitor for code recurrence during a complete heat cycle.

For used excavators: Always inspect coolant condition during this repair—contaminated or degraded coolant accelerates sensor failure. Check for electrolysis in the cooling system, which can damage sensors and wiring over time.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Hyundai service manual for your specific machine serial number, and consider professional diagnostic assistance for complex electrical issues or if you're uncertain about any procedure.