What is Hyundai R335LC-9T Fault Code 105-4?

Fault Code 105-4 on the Hyundai R335LC-9T excavator indicates an Intake Air Temperature (IAT) Sensor circuit voltage reading is too high (open circuit or short to positive voltage). This code is triggered when the Engine Control Module (ECM) detects a voltage signal from the intake air temperature sensor that exceeds the normal operating range (typically above 4.5-4.8 volts on a 5-volt reference circuit).

The IAT sensor is a critical thermistor-type component located in the intake manifold that measures incoming air temperature. The ECM uses this data to adjust fuel injection timing, air-fuel ratio, and turbocharger boost pressure for optimal combustion efficiency. On the Hyundai R335LC-9T's Cummins QSB6.7 engine, accurate intake air temperature readings are essential for maintaining emissions compliance, preventing excessive exhaust gas temperatures, and ensuring proper engine protection strategies activate when needed.

Common Symptoms

When Code 105-4 is active, operators typically experience:

• Check Engine Light (CEL) or malfunction indicator lamp illuminated on the instrument cluster
• Reduced engine power or derate mode (engine may limit to 70-80% power output)
• Rough idle or hard starting, especially in cold ambient conditions
• Black smoke from exhaust due to improper air-fuel mixture calculations
• Poor fuel economy as the ECM defaults to conservative fuel mapping when IAT data is unreliable

Potential Causes

The most common technical reasons for Code 105-4 on used R335LC-9T excavators include:

• Open circuit in the IAT sensor wiring harness (broken wire or poor connection at the sensor plug)
• Corroded or damaged connector pins at the sensor itself—common wear point on machines operating in wet or corrosive environments
• Failed IAT sensor with internal circuit failure causing infinite resistance reading
• Wiring harness chafing against engine components near the intake manifold—a known issue on higher-hour machines where vibration causes harness movement
• Short to battery voltage from damaged insulation contacting the intake manifold heater circuit or other powered circuits
• ECM internal failure (rare, but possible on machines with electrical system damage history)

How to Troubleshoot and Fix Code 105-4

Step 1: Visual Inspection Begin by inspecting the IAT sensor connector located on the intake manifold between the turbocharger and intake ports. Disconnect the two-pin connector and examine both male sensor pins and female harness terminals for corrosion, bent pins, or moisture intrusion. On used excavators, this connection point frequently suffers from heat cycling and environmental contamination. Clean contacts with electrical contact cleaner and ensure the rubber seal is intact.

Step 2: Sensor Resistance Testing Using a digital multimeter, measure the resistance across the IAT sensor terminals (sensor disconnected). At 68°F (20°C), resistance should typically read 2,200-2,800 ohms (consult service manual for exact specification). An infinite or very high resistance (above 10K ohms) confirms sensor failure. Also measure resistance while gently moving the sensor body—erratic readings indicate internal sensor damage common in high-vibration applications.

Step 3: Circuit Voltage Testing With the sensor disconnected and ignition key ON (engine off), measure voltage at the harness connector. You should see approximately 5 volts on the signal wire (referenced to ground). If voltage reads above 4.8 volts continuously, suspect an open ground circuit or damaged signal wire. Check continuity between the sensor ground pin and chassis ground—should read less than 1 ohm resistance.

Step 4: Harness Inspection for Wear Points On R335LC-9T machines with over 5,000 hours, physically trace the sensor harness from the intake manifold back to the main engine harness connection point. Pay special attention where the harness passes near the intake manifold bolts and fuel rail mounting brackets—these are common rub-through points. Look for wire insulation damage that could cause intermittent shorts to voltage when the harness moves during operation.

Step 5: Software Diagnosis and Verification Connect Hyundai diagnostic software (Hi-MATE) or compatible Cummins Insite software to verify real-time IAT sensor readings. With sensor connected in ambient temperature, displayed temperature should match actual air temperature within 5°F. If the display shows implausible temperatures (below -40°F or above 300°F when engine is cold), this confirms circuit fault rather than sensor calibration issue.

After repairs, clear the fault code, run the engine through several operating temperature cycles, and verify the code does not return. For used equipment buyers, persistent Code 105-4 after sensor and harness replacement may indicate deeper ECM connector corrosion at the main engine harness connection—a more expensive repair requiring significant labor for connector access and potential ECM replacement.

Disclaimer: This guide provides general troubleshooting information. Always consult the manufacturer's service manual for your specific machine serial number and consult with a certified Hyundai excavator technician for complex electrical diagnostics or emissions-related repairs.