What is Hyundai R335LC-9T Fault Code 122?

Fault Code 122 on the Hyundai R335LC-9T excavator indicates a "Boost 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 turbocharger boost pressure sensor, typically below the manufacturer's specified threshold of approximately 0.5 volts.

This sensor plays a critical role in monitoring intake manifold pressure, allowing the ECM to optimize fuel injection timing, air-fuel ratios, and turbocharger performance. On the R335LC-9T's Cummins QSB6.7 or similar tier-compliant engine, accurate boost pressure readings are essential for maintaining proper power output, fuel efficiency, and emissions compliance. When this code activates, the ECM may enter a derate mode to protect the engine from potential damage caused by incorrect fuel mapping or over-boosting conditions.

Common Symptoms

• Engine derate or reduced power output, particularly noticeable during high-load operations like digging or lifting
• Check Engine Light (CEL) or malfunction indicator lamp illuminated on the instrument cluster
• Black smoke from exhaust due to improper air-fuel mixture calculations
• Rough idle or hesitation during acceleration as the ECM compensates for missing sensor data
• Fuel economy decline as the system defaults to conservative fuel delivery parameters

Potential Causes

The most common technical causes for Code 122 on used R335LC-9T excavators include:

• Damaged or corroded wiring harness between the boost pressure sensor and ECM, particularly at routing points near the engine block where heat and vibration cause insulation breakdown
• Failed boost pressure sensor due to internal component degradation, contamination from oil mist, or moisture intrusion
• Poor electrical connections at the sensor connector or ECM pins, often caused by corrosion or loose terminal contacts in high-hour machines
• Shorted sensor ground circuit or broken reference voltage wire (typically 5V supply from ECM)
• ECM internal fault affecting sensor input circuitry (less common but possible in older units)

How to Troubleshoot and Fix Code 122

Step 1: Visual Inspection Begin by locating the boost pressure sensor on the intake manifold. Inspect the sensor connector and wiring harness for obvious damage, corrosion, oil contamination, or chafing against engine components. On used excavators, pay special attention to harness routing near vibration points and heat sources. Clean all connector pins with electrical contact cleaner and ensure positive terminal engagement.

Step 2: Electrical Testing Using a digital multimeter (DMM), perform voltage and resistance checks. With ignition on and engine off, verify 5-volt reference supply at the sensor connector (typically pin A). Check sensor ground circuit continuity to chassis ground (should read less than 1 ohm resistance). Measure sensor output voltage—at atmospheric pressure, it should read approximately 0.5-1.0 volts. Any reading below 0.3 volts indicates sensor or wiring failure.

Step 3: Component Testing and Replacement Disconnect the sensor and test its internal resistance according to Hyundai specifications (typically 1,000-3,000 ohms depending on temperature). If readings are out of range or the sensor shows no response to applied pressure, replace the boost pressure sensor with an OEM or quality aftermarket unit. After installation, clear codes using Hyundai Hi-MATE diagnostic software or a compatible scan tool, then perform a functional test under load to verify proper operation.

Disclaimer: This guide provides general diagnostic information. Always consult the official Hyundai service manual for your specific machine serial number. If you're unfamiliar with electrical diagnostics or high-pressure systems, seek assistance from a qualified heavy equipment technician to avoid injury or further damage.