XCMG QSX15 Fault Code 108-4: Complete Diagnostic Guide

What is XCMG QSX15 Fault Code 108-4?

Fault Code 108-4 indicates a "Barometric Pressure Sensor Circuit - Voltage Below Normal or Shorted to Low Source" condition in the XCMG excavator equipped with a Cummins QSX15 engine. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormally low voltage signal from the barometric pressure sensor, typically below 0.5 volts when the normal operating range is 4.5-5.0 volts.

The barometric pressure sensor is critical for the QSX15's fuel injection timing and air-fuel ratio calculations. The ECM uses atmospheric pressure data to compensate for altitude changes and ensure optimal combustion efficiency. When this sensor circuit fails, the engine defaults to a predetermined sea-level value, potentially causing performance degradation, especially at higher elevations or varying work site altitudes.

Common Symptoms

When Code 108-4 is active on your XCMG excavator, you may experience:

• Check Engine Light (CEL) or Malfunction Indicator Lamp (MIL) illuminated on the instrument cluster
• Engine power derate or reduced throttle response, particularly noticeable at higher elevations
• Black smoke from exhaust due to improper fuel compensation
• Rough idle or hesitation during acceleration under load
• Increased fuel consumption as the ECM operates with default atmospheric pressure values

Potential Causes

The most common technical failures causing Code 108-4 in used XCMG QSX15 excavators include:

• Damaged or corroded sensor connector – particularly common on machines operating in coastal or high-humidity environments
• Shorted or chafed wiring harness – check routing near the engine block where vibration causes insulation wear
• Failed barometric pressure sensor – internal component degradation in high-hour machines (typically 8,000+ hours)
• ECM internal circuit failure – rare but possible in older control modules
• Poor ground connection at sensor mounting location due to rust or paint buildup

How to Troubleshoot and Fix Code 108-4

Step 1: Visual Inspection Begin by locating the barometric pressure sensor (typically integrated into the ECM housing or mounted separately near the intake manifold on QSX15 engines). Inspect the three-pin connector for corrosion, bent pins, moisture intrusion, or damaged seals. On used excavators, check for oil contamination from valve cover leaks that may have compromised the connector integrity.

Step 2: Electrical Testing Using a digital multimeter (DMM), perform a voltage check with the key in the ON position (engine off). Backprobe the sensor's 5-volt reference wire (typically the red or orange wire) – you should read 4.5-5.0 volts. Check the signal wire for continuity to the ECM and verify the ground circuit has less than 0.1 ohms resistance to chassis ground.

Step 3: Sensor and Harness Verification Disconnect the sensor and measure resistance across the sensor terminals per Cummins specifications (typically 1,000-3,000 ohms at 68°F). If readings are out of range, replace the sensor. For used machines, carefully trace the wiring harness from sensor to ECM, checking for chafe points against the engine block, frame rails, or hydraulic lines – a common failure point after 5,000+ operating hours.

Step 4: Clear Code and Test After repairs, use Cummins INSITE diagnostic software or a compatible scan tool to clear the fault code. Perform a road test under load and monitor live data to confirm the barometric pressure reading matches local atmospheric conditions (approximately 14.7 psi at sea level).

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Disclaimer: This guide provides general diagnostic information for Code 108-4. Always consult the official XCMG and Cummins service manuals for your specific machine serial number. If you're uncomfortable performing electrical diagnostics, contact a certified Cummins or XCMG technician to prevent further damage to engine control systems.