What is Hyundai R335LC-9T Fault Code 655-5?

Fault Code 655-5 on the Hyundai R335LC-9T excavator indicates an Exhaust Gas Recirculation (EGR) Valve Position Sensor Circuit Low Voltage condition. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects that the voltage signal from the EGR valve position sensor has fallen below the manufacturer's specified threshold, typically below 0.5 volts during operation.

The EGR system is critical for emissions compliance and engine efficiency on this Tier 4 Final compliant machine. The position sensor provides real-time feedback to the ECM about the EGR valve's actual opening percentage, allowing precise control of exhaust gas recirculation rates. When this sensor circuit reports abnormally low voltage, the ECM cannot accurately control NOx emissions, potentially causing the engine to enter derate mode or fail emissions standards.

Common Symptoms

• Yellow or amber engine warning light illuminated on the instrument cluster, sometimes accompanied by a check engine symbol
• Reduced engine power (derate) limiting machine productivity, particularly under heavy load conditions
• Rough idle or hesitation during acceleration due to improper EGR flow control
• Increased black smoke from the exhaust during operation, indicating combustion inefficiency
• DPF regeneration cycles occurring more frequently than normal due to improper air-fuel mixture

Potential Causes

The most common technical reasons for Code 655-5 on used R335LC-9T excavators include:

• Damaged or corroded wiring harness near the EGR valve assembly, particularly where the harness contacts the engine block or routing brackets (common wear point on high-hour machines)
• Failed EGR position sensor due to carbon buildup, heat exposure, or internal electrical component degradation
• Poor ground connection at the sensor or ECM, often caused by corrosion on terminals in older equipment
• Chafed wiring at flex points between the engine and chassis, especially on machines operating in harsh environments
• Water intrusion into connector terminals from damaged seals or improper pressure washing practices
• ECM internal fault (rare, but possible in high-hour machines with electrical system history)

How to Troubleshoot and Fix Code 655-5

Step 1: Visual Inspection Begin with a thorough examination of the EGR valve position sensor wiring harness from the sensor to the ECM. Look specifically for rubbed-through insulation, crushed wires near mounting points, corroded connectors, or evidence of previous repairs. On used excavators, check for aftermarket wire repairs or electrical tape indicating previous issues.

Step 2: Electrical Testing Using a digital multimeter (DVOM), disconnect the EGR position sensor connector and check for supply voltage at the harness side (typically 5 volts with key on, engine off). Then measure resistance across the sensor terminals (specification typically 1-6 kΩ depending on valve position). Check for continuity along each wire from sensor to ECM, and verify ground integrity with less than 0.5 ohms resistance to chassis ground.

Step 3: Sensor and Component Testing If wiring checks pass, monitor the sensor signal voltage using Hyundai Hi-MATE diagnostic software or equivalent while manually actuating the EGR valve. The voltage should sweep smoothly from approximately 0.5V to 4.5V. Erratic readings or stuck values indicate sensor failure. Remove the sensor and inspect for carbon deposits or physical damage that could affect the internal potentiometer.

Step 4: Connector Service For used equipment, even if connectors appear intact, disconnect and clean all terminals with electrical contact cleaner, inspect pins for corrosion or spreading, and apply dielectric grease before reconnecting. This simple step resolves many intermittent sensor codes on older machines.

Step 5: Component Replacement Replace the EGR position sensor if electrical tests confirm sensor failure. On high-hour machines, consider replacing both the sensor and the mating connector pigtail as preventive maintenance, as terminal corrosion often reoccurs.

Disclaimer: This guide provides general troubleshooting information for educational purposes. Always consult the official Hyundai service manual for your specific serial number, and consider engaging a certified Hyundai technician for complex diagnostics. Improper repairs may affect emissions compliance and machine safety.