What is Hyundai HX220 NL EPFC Fault Code 351 (627-12)?

Fault Code 351 (627-12) indicates a "Hydraulic Oil Temperature Sensor Circuit Low Voltage" condition in the Hyundai HX220 NL EPFC excavator. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormally low voltage signal from the hydraulic oil temperature sensor circuit, typically below 0.5 volts for more than 2 seconds during operation.

This sensor is critical for monitoring hydraulic fluid temperature to prevent overheating damage to pumps, motors, and seals. The ECM uses this data to adjust engine performance and activate cooling fans. When the circuit voltage drops too low, the system cannot accurately monitor temperatures, potentially leading to component damage or reduced machine efficiency—especially problematic in used excavators with aging hydraulic systems.

Common Symptoms

• Hydraulic oil temperature warning light illuminated on the instrument cluster, even when oil temperature is normal
• Engine derate or power reduction mode activated as a protective measure
• Cooling fan running continuously at high speed regardless of actual hydraulic temperature
• Abnormal hydraulic system performance including sluggish operation or reduced lifting capacity
• Error message displayed on the monitor panel indicating temperature sensor malfunction

Potential Causes

The most common technical causes for Code 351 in used HX220 excavators include:

• Damaged or corroded wiring harness near the hydraulic tank mounting area where cable routing experiences vibration and abrasion
• Failed hydraulic oil temperature sensor due to internal short circuit or contamination from hydraulic fluid degradation
• Corroded or loose connector pins at the sensor plug, especially on machines operated in wet or marine environments
• Chafed wires rubbing against the tank frame or hydraulic lines—a known wear point on HX220 models
• ECM internal fault or poor ground connection at the control module (less common)
• Previous repair attempts using incorrect sensor specifications or aftermarket components

How to Troubleshoot and Fix Code 351 (627-12)

Step 1: Visual Inspection Locate the hydraulic oil temperature sensor mounted on the hydraulic oil tank. Inspect the wiring harness from sensor to ECM for visible damage, chafing, or oil contamination. On used excavators, carefully check connector terminals for corrosion or bent pins—clean with electrical contact cleaner if necessary.

Step 2: Electrical Testing Disconnect the sensor connector. Using a digital multimeter, measure resistance across the sensor terminals. A functional sensor typically reads 2,000-3,000 ohms at room temperature. Check for short to ground by measuring resistance between each sensor terminal and machine ground—it should read infinite (open circuit).

Step 3: Circuit Voltage Check With ignition on and sensor disconnected, measure voltage at the harness connector from the ECM side. You should see approximately 5 volts reference voltage. If voltage is absent or below 4.5V, trace the circuit back toward the ECM checking for damaged wires or poor connections.

Step 4: Sensor Replacement and Verification If testing confirms sensor failure, replace with OEM-specification parts. For used machines, apply dielectric grease to connector pins before reassembly to prevent future corrosion. Clear codes using Hyundai Hi-MATE diagnostic software or compatible scan tool and verify repair by operating machine through full temperature range.

Disclaimer: This guide provides general troubleshooting information. Hydraulic and electrical system repairs should be performed by qualified technicians following manufacturer service procedures. Always consult the official Hyundai service manual and consider machine-specific history when diagnosing used equipment.