What is XCMG QSB6.7 CM2150 B109 Fault Code 122?

Fault Code 122 indicates an Intake Manifold Pressure (Boost Pressure) Sensor Circuit Low Voltage condition on the Cummins QSB6.7 CM2150 engine used in XCMG excavators. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects a voltage signal from the intake manifold pressure sensor that falls below the manufacturer's specified minimum threshold, typically below 0.5 volts.

The intake manifold pressure sensor is critical for proper engine performance because it measures boost pressure from the turbocharger and atmospheric pressure. The ECM uses this data to calculate optimal fuel injection timing, air-fuel ratios, and turbocharger control. When this sensor circuit reports abnormally low voltage, the engine cannot accurately manage combustion efficiency, potentially leading to power loss, increased emissions, and engine protection modes activating.

Common Symptoms

• Check Engine Light or Malfunction Indicator Lamp (MIL) illuminated on the dashboard
• Noticeable loss of engine power or reduced hydraulic performance during digging operations
• Engine may enter derate mode (limited RPM or torque output) to prevent damage
• Black smoke from exhaust due to incorrect fuel-to-air ratio calculations
• Rough idling or hesitation during acceleration under load

Potential Causes

The most common technical reasons for Code 122 on used XCMG excavators include:

• Damaged or corroded wiring harness between the intake manifold pressure sensor and ECM (especially at connection points near the engine block where vibration and heat cause wear)
• Faulty intake manifold pressure sensor due to internal circuit failure or contamination from oil vapor
• Corroded or loose electrical connectors at the sensor or ECM—common in machines exposed to harsh environments
• Shorted sensor signal wire to ground caused by harness rubbing against frame or engine components
• ECM internal circuit failure (rare, but possible in high-hour used machines)
• Poor ground connection at the sensor mounting point or ECM chassis ground

How to Troubleshoot and Fix Code 122

Step 1: Visual Inspection Begin by inspecting the intake manifold pressure sensor (typically located on the intake manifold near the turbocharger outlet). Check for physical damage, oil contamination, or loose mounting. Examine the wiring harness from sensor to ECM for abrasion, cuts, or signs of rubbing against hot or moving components—critical in used excavators with high operating hours.

Step 2: Connector and Ground Testing Disconnect the sensor's 3-pin connector and inspect for corrosion, bent pins, or moisture intrusion. Clean contacts with electrical contact cleaner if needed. Using a digital multimeter (DMM), verify the ground circuit by checking continuity between the sensor ground pin and a known good chassis ground (should read less than 5 ohms resistance).

Step 3: Voltage Supply Check With the ignition ON and sensor disconnected, measure voltage at the 5-volt reference supply wire at the sensor connector (consult wiring diagram for pin identification). You should read approximately 4.5 to 5.2 volts. If voltage is absent or incorrect, trace the circuit back to the ECM for breaks or shorts.

Step 4: Sensor Resistance Testing Measure resistance across the sensor terminals with the sensor disconnected. A functioning sensor typically shows resistance values between 1,000 to 3,000 ohms (verify exact specifications in service manual). Values significantly outside this range indicate sensor failure.

Step 5: Signal Wire Integrity Check the signal wire for shorts to ground by measuring resistance between the signal pin (at the ECM side, sensor disconnected) and chassis ground. Reading should be infinite (open circuit). Any continuity indicates a short requiring harness repair or replacement.

Step 6: Component Replacement If all circuits test correctly but the code persists, replace the intake manifold pressure sensor with a genuine Cummins or OEM-equivalent part. After replacement, clear codes using Cummins INSITE diagnostic software or compatible scan tool, and perform a test run under load to verify repair.

Critical Note for Used Excavators: Before replacing expensive components, thoroughly inspect all connector seals and harness routing. Years of vibration often cause intermittent shorts that are easily remedied with proper harness securing and connector resealing.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official XCMG and Cummins service manuals for your specific machine model. Complex electrical diagnostics should be performed by qualified diesel technicians with appropriate diagnostic equipment.