What is Hyundai HX220 NL EPFC Fault Code 135-2?

Fault Code 135-2 on the Hyundai HX220 NL EPFC indicates a malfunction in the Engine Coolant Temperature (ECT) Sensor Circuit – specifically a low voltage or short-to-ground condition. This code is triggered when the Electronic Control Module (ECM) detects an abnormally low voltage signal from the coolant temperature sensor, typically below 0.2 volts when normal operating range should be 0.5-4.5 volts.

The ECT sensor plays a critical role in the HX220's engine management system by monitoring coolant temperature and feeding this data to the ECM. The ECM uses this information to adjust fuel injection timing, idle speed, and emissions control strategies. When this sensor circuit fails, the ECM cannot properly manage engine temperature, potentially leading to poor fuel economy, excessive emissions, and inadequate engine protection during cold starts or overheating conditions.

Common Symptoms

• Check Engine Light or malfunction indicator illuminated on the dashboard display
• Hard starting or rough idle, especially during cold engine starts
• Black smoke from exhaust due to overly rich fuel mixture
• Engine cooling fan running continuously at high speed, even when engine is cold
• Reduced engine performance or power derate mode activation to protect the engine
• Abnormal fuel consumption as the ECM defaults to a failsafe temperature value

Potential Causes

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

• Damaged or corroded wiring harness near the thermostat housing where vibration and heat cause insulation breakdown
• Faulty ECT sensor with internal short circuit or contaminated sensing element
• Corroded or loose electrical connectors at the sensor or ECM connection points (common in machines exposed to water or mud)
• Chafed wiring along the engine block where the harness contacts metal surfaces during engine movement
• Failed ECM internal circuits (less common but possible in high-hour machines)
• Coolant contamination on sensor terminals causing electrical shorts

How to Troubleshoot and Fix Code 135-2

Step 1: Visual Inspection Begin by inspecting the ECT sensor located near the thermostat housing on the engine block. Check for coolant leaks, corrosion on the connector, or physical damage. On used excavators, carefully trace the sensor wiring harness along its entire length, looking for worn insulation, rub points against engine components, or previous repair attempts with electrical tape.

Step 2: Electrical Testing Disconnect the ECT sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At room temperature (68°F/20°C), resistance should be approximately 2,000-3,000 ohms. Compare readings against Hyundai's specification chart. Next, check for continuity to ground on the sensor signal wire with the harness disconnected – any continuity indicates a short circuit in the wiring.

Step 3: Voltage and Harness Testing With the ignition on and sensor disconnected, measure voltage at the ECM supply wire (typically 5 volts). If voltage is absent or incorrect, trace the harness back to the ECM checking for breaks or shorts. Inspect all connector pins for corrosion, bent terminals, or moisture intrusion – common issues in older machines.

Step 4: Component Replacement If testing confirms a faulty sensor, replace it with an OEM or high-quality aftermarket ECT sensor and apply dielectric grease to connector terminals. If wiring is damaged, repair using proper gauge wire and heat-shrink connections rather than tape. Clear the fault code using Hyundai diagnostic software or a compatible scan tool and verify the repair during a test run.

Disclaimer: This guide provides general troubleshooting information. Always consult the Hyundai HX220 service manual for exact specifications and safety procedures. If you're uncomfortable performing electrical diagnostics, consult a certified heavy equipment technician to prevent further damage to the machine's electrical system.