Hyundai HX220 NL EPFC Fault Code 3737 (1675-31): Meaning and Fix

What is Hyundai HX220 NL EPFC Fault Code 3737 (1675-31)?

Fault Code 3737 (1675-31) indicates a malfunction in the engine coolant temperature sensor circuit, specifically detecting an abnormal voltage signal or open circuit condition in the ECM (Engine Control Module) monitoring system.

This code triggers when the ECM detects that the coolant temperature sensor is sending voltage readings outside the expected range (typically 0.5-4.5V). The engine coolant temperature sensor is critical for the HX220 NL's EPFC (Electronic Pump Flow Control) system, as it directly influences fuel injection timing, hydraulic pump output, and engine protection protocols. When this sensor fails, the ECM cannot accurately manage engine temperature, potentially leading to overheating, reduced hydraulic performance, or engine derate modes.

Common Symptoms

• Engine warning light illuminated on the instrument cluster, often accompanied by a derate notification
• Reduced hydraulic power or sluggish boom/arm response due to ECM limiting pump flow as a protective measure
• Hard starting or rough idle, especially when the engine is cold
• Cooling fan running continuously at high speed, even when coolant temperature appears normal
• Intermittent stalling or unexpected engine shutdown during operation

Potential Causes

• Corroded or damaged sensor connector – Common on used excavators due to exposure to moisture, dirt, and vibration in the engine bay
• Failed coolant temperature sensor – Internal resistance drift or complete sensor failure after years of thermal cycling
• Wiring harness damage – Chafing against engine components or frame rails, particularly near the thermostat housing where the harness routes
• Poor ground connection at the sensor or ECM, causing erratic voltage readings
• ECM internal fault – Rare, but possible on high-hour machines with previous electrical issues or water intrusion

How to Troubleshoot and Fix Code 3737 (1675-31)

Step 1: Visual Inspection
Locate the coolant temperature sensor on the thermostat housing (driver's side of engine block). Inspect the wiring harness for visible damage, abrasion, or oil contamination. Check the connector for corrosion, bent pins, or moisture intrusion – a common issue on used excavators operating in wet conditions.

Step 2: Electrical Testing
Disconnect the sensor connector. Using a digital multimeter, measure resistance across the sensor terminals. At 68°F (20°C), resistance should read approximately 2,000-3,000 ohms (consult Hyundai service manual for exact specs). If the reading is infinite (open circuit) or near zero (short circuit), replace the sensor.

Step 3: Circuit Voltage Check
With the ignition on and sensor disconnected, measure voltage at the harness connector. You should see approximately 5V reference voltage from the ECM. If voltage is absent or significantly lower, inspect the harness for breaks or shorts to ground between the sensor and ECM.

Step 4: Connector and Ground Verification
Clean all connector terminals with electrical contact cleaner and apply dielectric grease to prevent future corrosion. Verify the ground circuit has less than 0.5 ohms resistance to chassis ground. On used machines, ground points often corrode, causing intermittent faults.

Step 5: Clear Code and Test
After repairs, clear the fault code using Hyundai Hi-MATE diagnostic software or a compatible scan tool. Run the engine through multiple temperature cycles and monitor for code recurrence.

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Disclaimer: This guide provides general troubleshooting steps. For complex electrical diagnostics or if the fault persists after following these procedures, consult a certified Hyundai heavy equipment technician or authorized service center. Improper repairs may void warranties or cause additional damage.