What is Hyundai R455LC-7 Fault Code 108-4?

Fault Code 108-4 on the Hyundai R455LC-7 excavator indicates a Barometric Pressure Sensor Circuit Low Voltage condition. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormally low voltage signal from the barometric pressure sensor, which falls below the manufacturer's specified threshold of approximately 0.5 volts.

The barometric pressure sensor plays a critical role in engine performance by measuring atmospheric pressure and sending this data to the ECM. The ECM uses this information to adjust fuel injection timing, air-fuel ratios, and turbocharger boost levels to compensate for altitude changes. When Code 108-4 appears, the ECM cannot accurately calculate air density, leading to suboptimal combustion efficiency, reduced power output, and potentially increased emissions on this Cummins QSX15-powered machine.

Common Symptoms

When Code 108-4 is active on your R455LC-7, you may experience:

• Engine warning light illuminated on the instrument cluster with reduced engine power
• Derate mode activation, limiting engine RPM to approximately 1,500-1,800 RPM
• Difficulty starting or rough idle, especially at higher elevations
• Black smoke from the exhaust due to improper fuel mixture calculations
• Noticeable loss of hydraulic performance during heavy digging operations

Potential Causes

The most common technical reasons for Code 108-4 on used R455LC-7 excavators include:

• Damaged or corroded wiring harness between the barometric pressure sensor and ECM, particularly at known rub points near the engine valley cover
• Faulty barometric pressure sensor due to internal circuit failure or moisture ingress
• Corroded or loose electrical connectors at the sensor or ECM interface
• Shorted sensor ground wire causing voltage drop in the signal circuit
• Failed ECM (rare, but possible in high-hour machines with 8,000+ operating hours)

How to Troubleshoot and Fix Code 108-4

Step 1: Visual Inspection Begin by locating the barometric pressure sensor (typically mounted on the engine valve cover or intake manifold). Inspect the wiring harness for physical damage, chafing, or oil contamination. On used excavators, pay special attention to areas where harnesses route near hot exhaust components or moving engine parts.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor connector and measure reference voltage from the ECM. You should see approximately 5 volts on the reference pin with the key on, engine off. Check ground continuity (less than 5 ohms resistance to chassis ground) and inspect for pin corrosion inside connector terminals.

Step 3: Sensor Testing and Replacement With the sensor connected, backprobe the signal wire while monitoring voltage. At sea level, expect 4.5-4.8 volts; readings below 0.5 volts confirm sensor failure. If wiring tests pass but voltage remains low, replace the barometric pressure sensor (Hyundai part number 21634246 or equivalent). Clear codes using Hyundai Hi-MATE diagnostic software and perform a test cycle.

Step 4: Advanced Diagnostics If symptoms persist after sensor replacement, verify ECM power supply circuits and consider reflashing the ECM software to the latest calibration. For used machines, inspect the main engine harness for historical repairs that may have compromised circuit integrity.

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Disclaimer: This guide provides general troubleshooting information. Always consult your Hyundai service manual and consider professional diagnostic assistance for complex electrical issues. Improper repairs may void warranties or cause additional damage.