What is Hyundai HX220 NL EPFC Fault Code 1633 (625-2)?

Fault Code 1633 (625-2) indicates a malfunction in the engine coolant temperature sensor circuit, specifically pointing to an abnormal voltage signal or sensor range problem within the Engine Control Module (ECM) monitoring system. This code triggers when the coolant temperature sensor sends readings outside the acceptable parameters defined by Hyundai's electronic fuel injection system, typically indicating either an open circuit, short circuit, or sensor degradation.

For the HX220 NL EPFC model, this fault is critical because the ECM relies on accurate coolant temperature data to optimize fuel injection timing, manage engine load, and prevent overheating during heavy excavation work. Ignoring this code can lead to poor fuel economy, reduced hydraulic performance, and potential engine damage from improper temperature management.

Common Symptoms

• Engine warning light illuminated on the instrument cluster, often accompanied by reduced engine power
• Hard starting conditions, especially during cold weather, due to incorrect fuel enrichment calculations
• Erratic engine idle or rough running as the ECM defaults to preset temperature values
• Black smoke from exhaust during acceleration, indicating improper fuel-air mixture compensation
• Activation of engine derate mode, limiting machine performance to protect components from thermal damage

Potential Causes

The most common causes for Code 1633 in used HX220 excavators include:

• Corroded or damaged wiring harness near the coolant sensor connector, particularly where the harness contacts the engine block or frame rails
• Failed coolant temperature sensor due to age-related internal resistance drift or thermal cycling damage
• Loose or contaminated electrical connections at the sensor plug, often caused by coolant leaks or moisture intrusion
• Broken sensor mounting threads allowing coolant seepage into the electrical terminals
• ECM internal fault (less common) affecting the temperature monitoring circuit

How to Troubleshoot and Fix Code 1633 (625-2)

Step 1: Visual Inspection Begin by locating the coolant temperature sensor on the engine block (typically near the thermostat housing). Inspect the wiring harness for visible damage, chafing against metal surfaces, or signs of heat damage. Check the connector for corrosion, bent pins, or coolant contamination—common issues in used excavators operating in harsh environments.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance. At room temperature (20°C/68°F), resistance should typically read between 2,000-3,000 ohms (consult your service manual for exact specifications). Test the harness voltage with the ignition on—you should see approximately 5 volts reference voltage from the ECM. Any reading below 4.5V or above 5.5V indicates wiring or ECM issues.

Step 3: Component Replacement and Verification If the sensor tests outside specifications, replace it with a genuine Hyundai coolant temperature sensor. Before installation, clean the mounting threads and apply appropriate thread sealant. For used machines, inspect the sensor bore for thread damage or coolant passage blockages. After replacement, clear the fault code using Hyundai diagnostic software (Hi-MATE or equivalent) and perform a cold-start test cycle to verify proper operation.

Disclaimer: This guide provides general troubleshooting procedures. Always consult your machine's official service manual and consider professional diagnostic assistance for complex electrical issues or if you're unfamiliar with high-voltage systems.