What is XCMG QSB4.5 CM2150 B108 Fault Code 122?

Fault Code 122 indicates an Intake Manifold Pressure (Boost Pressure) Sensor Circuit Low Voltage condition on the XCMG excavator equipped with a Cummins QSB4.5 CM2150 engine. This diagnostic trouble code triggers when the Engine Control Module (ECM) detects that the voltage signal from the intake manifold pressure sensor has dropped below the acceptable threshold, typically less than 0.5 volts.

The intake manifold pressure sensor is critical for this turbocharged diesel engine's performance. It measures boost pressure generated by the turbocharger and sends this data to the ECM, which then adjusts fuel delivery and timing for optimal combustion. When Code 122 is active, the ECM loses accurate pressure readings, forcing the engine into a protective derate mode that significantly reduces available power and can compromise jobsite productivity.

Common Symptoms

• Check Engine Light (CEL) or malfunction indicator illuminated on the instrument cluster
• Engine power derate – noticeable reduction in horsepower, typically 25-40% loss, making heavy digging or lifting difficult
• Poor throttle response and sluggish acceleration when operating under load
• Black smoke from the exhaust due to improper air-fuel ratio calculations
• Rough idling or hesitation during operation, especially during cold starts

Potential Causes

The most common technical failures causing Code 122 on used XCMG excavators include:

• Damaged or corroded sensor wiring harness – particularly at rub points near the engine block or valve cover where vibration causes insulation wear
• Failed intake manifold pressure sensor – internal circuit failure or moisture intrusion in older sensors (common after 5,000+ hours)
• Loose or corroded electrical connectors at the sensor or ECM – corrosion from coolant leaks or environmental exposure
• Shorted sensor ground wire to engine block or chassis
• ECM internal fault – less common but possible in high-hour machines with electrical system history

How to Troubleshoot and Fix Code 122

Step 1: Visual Inspection Locate the intake manifold pressure sensor (mounted on the intake manifold near the turbocharger outlet). Inspect the three-pin connector for corrosion, bent pins, or moisture. On used excavators, check the wiring harness routing for wear points where it contacts the engine – this is a known issue on CM2150 engines with 3,000+ operating hours.

Step 2: Electrical Testing Using a digital multimeter, backprobe the sensor connector with the ignition on, engine off. Measure the 5-volt reference supply from the ECM (should read 4.5-5.2V). Check the sensor ground circuit for continuity to chassis ground (should be less than 0.5 ohms resistance). If voltage is absent or ground shows high resistance, trace and repair the harness before replacing the sensor.

Step 3: Sensor Verification If circuits test correctly, disconnect the sensor and measure resistance across the signal and ground pins. Compare readings to Cummins specifications (typically 1,000-3,000 ohms at atmospheric pressure). Replace the sensor if readings are out of range. For used machines, always apply dielectric grease to connector pins during reassembly to prevent future corrosion.

Step 4: Clear Codes and Test After repairs, use Cummins INSITE software or compatible diagnostic tool to clear fault codes. Perform a road test under load conditions and monitor live boost pressure data to verify proper sensor operation (should read 0-35 PSI depending on throttle position).

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Disclaimer: This guide provides general troubleshooting information. Always consult the official XCMG service manual and Cummins QuickServe documentation for your specific engine serial number. If unfamiliar with high-pressure fuel systems or electrical diagnostics, seek assistance from a certified Cummins technician.