What is Hyundai HX220 NL EPFC Fault Code 585 (677-4)?

Fault Code 585 (677-4) indicates a malfunction in the Engine Pre-heater Control Circuit or Glow Plug System relay circuit on the Hyundai HX220 NL EPFC excavator. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormal voltage, open circuit, or short circuit in the pre-heating system that assists cold-start operations.

The pre-heater system is critical for diesel engine starting in cold weather conditions, especially on machines with significant operating hours. When this circuit fails, the ECM cannot properly control the glow plugs or intake air heater, potentially causing hard starts, excessive white smoke during cranking, and increased wear on starter components. For used HX220 excavators, this code often appears after years of thermal cycling and electrical harness degradation.

Common Symptoms

• Hard starting or extended cranking time, particularly in temperatures below 50°F (10°C)
• Check Engine Light (CEL) or warning lamp illuminated on the instrument cluster
• Excessive white or gray smoke during cold engine startup
• Rough idle for the first few minutes after starting
• Pre-heater indicator light may not illuminate during the key-on cycle, or may flash abnormally

Potential Causes

The most common technical causes for Code 585 on used Hyundai HX220 excavators include:

• Failed glow plug relay or intake air heater relay (common failure point after 5,000+ hours)
• Corroded or damaged wiring harness connectors near the engine block where heat and vibration cause deterioration
• Open circuit or short to ground in the pre-heater control wiring, often at harness rub points along the frame rail
• Faulty individual glow plugs causing excessive current draw that triggers protective circuit shutdown
• ECM internal driver circuit failure (less common, but possible on high-hour machines)
• Blown fuse in the pre-heater power supply circuit

How to Troubleshoot and Fix Code 585 (677-4)

Step 1: Visual Inspection and Connector Check Begin by inspecting all wiring harnesses and connectors associated with the glow plug system. On used excavators, focus on areas where harnesses contact the frame or engine block. Look for frayed wires, melted insulation, or green corrosion on connector pins. Clean all connectors with electrical contact cleaner and apply dielectric grease.

Step 2: Test Glow Plug Relay and Circuit Continuity Using a digital multimeter (DMM), locate the glow plug relay (typically in the main fuse/relay box). With the key off, remove the relay and test for continuity between relay terminals per the relay pin diagram. Check for battery voltage (12V) at the relay socket power supply terminal with key on. Verify ground circuit continuity (should read less than 0.5 ohms resistance).

Step 3: Individual Glow Plug Resistance Testing Disconnect the glow plug bus bar and measure the resistance of each glow plug individually. Typical specification is 0.5-2.0 ohms. Any glow plug reading open circuit (infinite resistance) or significantly different from others should be replaced. Check for shorts to ground by measuring resistance between each glow plug terminal and engine block (should read infinite).

Step 4: ECM Signal Wire Verification Using Hyundai Hi-Mate diagnostic software or a compatible scan tool, monitor the ECM pre-heater control output in live data while commanding the system on. Backprobe the control wire at the relay with your DMM to verify the ECM is sending the proper ground signal (should switch from 12V to 0V when commanded on).

Step 5: Harness Repair and Component Replacement For used excavators with high hours, pay special attention to harness wear points near the turbocharger and exhaust manifold where heat damage is common. Repair any damaged wiring using heat-shrink solder connectors rated for engine compartment temperatures. Replace the relay if testing confirms failure, and clear codes before retesting.

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Disclaimer: This guide provides general troubleshooting information for experienced technicians. Always consult the official Hyundai service manual for your specific serial number range and follow proper safety procedures. If unfamiliar with electrical diagnostics, consult a certified Hyundai heavy equipment technician to avoid damage to the ECM or wiring harness.