What is Hyundai R335LC-9T Fault Code 168-18?

Fault Code 168-18 on the Hyundai R335LC-9T excavator indicates a High Inlet Air Temperature (IAT) sensor voltage condition with a moderate severity level. This code is triggered when the Engine Control Module (ECM) detects an abnormally high voltage signal from the Intake Air Temperature sensor, typically reading above 4.5 volts, which corresponds to unrealistically low temperature readings or an open circuit condition.

The IAT sensor plays a critical role in engine performance by measuring incoming air temperature and allowing the ECM to adjust fuel injection timing and quantity accordingly. On the Hyundai R335LC-9T's Cummins QSB6.7 engine, accurate air temperature data is essential for maintaining optimal combustion efficiency, emissions control, and preventing excessive fuel consumption. When this code activates, the ECM enters a default mode using substitute values, which can significantly impact engine power and fuel economy.

Common Symptoms

• Check Engine Light or malfunction indicator lamp illuminated on the instrument cluster
• Noticeable loss of engine power or reduced throttle response, especially under heavy loads
• Increased black smoke from the exhaust during acceleration due to improper fuel-to-air ratio calculations
• Rough idle or hesitation during cold starts when accurate temperature compensation is most critical
• Increased fuel consumption as the ECM operates with default air temperature values rather than actual conditions

Potential Causes

The most common causes of Code 168-18 on used Hyundai R335LC-9T excavators include:

• Open circuit or broken wiring in the IAT sensor harness, particularly at flex points near the air intake manifold where vibration causes wire fatigue
• Corroded or loose connector pins at the IAT sensor plug—a frequent issue on machines operating in wet or coastal environments
• Failed IAT sensor with internal resistance breakdown, typically after 6,000+ operating hours
• Damaged sensor harness from rodent activity or rubbing against the engine block (common on stored machines)
• ECM-side wiring issues including damaged pins in the ECM connector or internal ECM faults (rare but possible on high-hour units)

How to Troubleshoot and Fix Code 168-18

Step 1: Visual Inspection Begin by locating the IAT sensor on the intake manifold or air inlet piping. Inspect the sensor connector for corrosion, bent pins, or moisture intrusion—particularly important on used excavators with outdoor storage history. Check the entire wiring harness for signs of chafing, cuts, or rodent damage at known rub points near engine mounts.

Step 2: Sensor Resistance Testing Disconnect the IAT sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At 68°F (20°C), resistance should read approximately 2,400-2,700 ohms (consult your service manual for exact specifications). Compare readings at different temperatures—resistance should decrease as temperature increases. If readings are infinite (open circuit) or significantly out of range, replace the sensor.

Step 3: Circuit Voltage Testing With the sensor disconnected and ignition ON (engine off), measure voltage at the harness-side connector. You should see approximately 5 volts on the signal wire (reference voltage from ECM) and continuity to ground on the ground wire. If voltage reads above 4.8 volts constantly, inspect for an open ground circuit or damaged signal return wire.

Step 4: Connector and Harness Repair For used machines, thoroughly clean connector terminals with electrical contact cleaner and apply dielectric grease to prevent future corrosion. Repair any damaged wiring with proper gauge wire and heat-shrink connections—avoid simple tape repairs on vibration-prone areas. If harness damage is extensive near known wear points, consider rerouting with protective loom.

Step 5: Clear Code and Verify After repairs, use Hyundai Hi-MATE diagnostic software or a compatible scan tool to clear the fault code. Operate the machine through various temperature conditions and load cycles to verify the repair. Monitor live IAT sensor data to ensure readings correspond appropriately to actual ambient temperatures (typically -40°F to 257°F range).

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Disclaimer: This guide provides general troubleshooting information for Code 168-18. Always consult the official Hyundai service manual for your specific machine serial number and software version. If you're uncomfortable with electrical diagnostics or the issue persists after following these steps, contact a certified Hyundai dealer or qualified heavy equipment technician for professional diagnosis and repair.