What is Hyundai HX220 NL EPFC Fault Code 707-1?

Fault Code 707-1 on the Hyundai HX220 NL EPFC indicates a Main Relief Valve Solenoid Circuit Malfunction, specifically reporting an open circuit or abnormal low voltage condition in the hydraulic pressure control system. This code is triggered when the Electronic Control Module (ECM) detects that the electrical signal to the main relief valve solenoid is outside the manufacturer's specified voltage range, typically below 2.0 volts when the system should be energized.

This fault is critical for the HX220 NL's performance because the main relief valve solenoid directly controls maximum system hydraulic pressure. When this circuit malfunctions, the excavator cannot properly regulate hydraulic pressure across all functions, potentially leading to unsafe operating conditions, reduced lifting capacity, and protection mode activation. The EPFC (Electronic Power Flow Control) system relies on precise solenoid operation to balance fuel efficiency with hydraulic performance.

Common Symptoms

• Hydraulic system derate with noticeably reduced digging force and slower cycle times across all functions
• Amber or red warning light illuminated on the instrument cluster, often accompanied by an audible alarm
• Erratic hydraulic response during combined operations (simultaneous boom and bucket movements)
• Intermittent loss of hydraulic power that may restore temporarily after machine restart
• Error message displayed on the monitor screen specifically referencing hydraulic system malfunction

Potential Causes

The most common causes of Code 707-1 on used HX220 NL excavators include:

• Damaged wiring harness near the main pump area where vibration and heat cause insulation breakdown over time
• Corroded or loose connector pins at the solenoid plug, especially on machines operating in wet or corrosive environments
• Failed main relief valve solenoid due to internal coil breakdown (typical after 6,000-8,000 operating hours)
• Chafed wiring at known rub points along the right-side frame rail where harnesses contact structural members
• ECM internal driver circuit failure (less common but possible on higher-hour machines)
• Low voltage supply from a weak battery or failing alternator affecting solenoid energization

How to Troubleshoot and Fix Code 707-1

Step 1: Visual Inspection and Connector Check Begin by locating the main relief valve solenoid on the main hydraulic pump assembly. Disconnect the solenoid connector and inspect for corrosion, bent pins, or moisture intrusion. On used excavators, this connector is prone to corrosion from hydraulic oil mist. Clean contacts with electrical contact cleaner and apply dielectric grease before reconnection.

Step 2: Solenoid Resistance Testing Using a digital multimeter (DMM), measure the solenoid coil resistance. Disconnect the solenoid and test between the two terminals. The specification for the HX220 NL main relief solenoid is typically 3-7 ohms at 20°C. If the reading shows infinite resistance (open circuit) or zero ohms (short circuit), replace the solenoid.

Step 3: Wiring Harness Continuity and Voltage Check With the connector still disconnected at the solenoid, check for continuity from the ECM connector (Pin 47 on the main harness) to the solenoid plug. Any reading above 2 ohms indicates harness damage. Next, turn the ignition to ON (engine off) and measure voltage at the solenoid connector while commanding the solenoid ON using Hyundai Hi-MATE diagnostic software or equivalent. You should see approximately 12 volts DC. No voltage indicates an ECM driver circuit failure or blown fuse.

Step 4: Inspect Known Wear Points For used machines, thoroughly inspect the harness routing along the right chassis rail and near the pump mounting bracket—these are documented rub points on HX220 models. Look for abraded insulation and repair with heat shrink tubing and proper harness securing.

Step 5: Test and Clear Code After repairs, clear the fault code using diagnostic software, operate the machine through full hydraulic cycles, and monitor for code recurrence. If the code returns immediately, suspect ECM issues requiring dealer-level diagnostics.

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Disclaimer: This guide provides general troubleshooting information for Hyundai HX220 NL EPFC excavators. Always consult the manufacturer's service manual for model-specific procedures and specifications. If you lack experience with hydraulic systems or electrical diagnostics, consult a certified heavy equipment technician to prevent injury or further machine damage.