What is Hyundai R335LC-9T Fault Code 171-4?

Fault Code 171-4 on the Hyundai R335LC-9T excavator indicates an Engine Coolant Temperature (ECT) Sensor Circuit voltage abnormality with severity level 4 (highest alert). This code specifically means the Engine Control Module (ECM) has detected a voltage reading outside the acceptable range from the coolant temperature sensor circuit, either abnormally high (open circuit) or low (short to ground).

This fault is critical for the R335LC-9T's Cummins QSB6.7 engine performance because the ECM relies on accurate coolant temperature data to control fuel injection timing, idle speed, and engine protection protocols. When this sensor circuit fails, the engine may enter derate mode to prevent potential overheating damage, significantly reducing machine productivity on job sites.

Common Symptoms

• Engine warning light illuminated on the instrument cluster with possible engine derate (reduced power output)
• Hard starting conditions, especially during cold weather, as the ECM cannot properly adjust fuel delivery
• Rough idle or irregular engine RPM due to incorrect temperature compensation
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Black smoke from exhaust caused by improper fuel-air mixture calibration

Potential Causes

The most common technical failures triggering Code 171-4 on used R335LC-9T excavators include:

• Damaged or corroded wiring harness at known rub points near the engine block mounting brackets or along the right side chassis rail
• Failed coolant temperature sensor (typical lifespan degradation after 6,000-8,000 operating hours)
• Corroded connector pins at the ECT sensor plug due to coolant weepage or moisture intrusion
• Broken sensor ground wire or poor ground connection at the engine block grounding point
• ECM internal circuit failure (rare, but documented in early 2014-2015 production models)

How to Troubleshoot and Fix Code 171-4

Step 1: Visual Inspection Locate the ECT sensor on the engine block (driver's side, near thermostat housing). Inspect the sensor connector for corrosion, coolant residue, or damaged pins. On used excavators, check the entire wiring harness route for chafing against engine components or frame rails—a common failure point after 5,000+ hours.

Step 2: Electrical Testing Disconnect the sensor and measure resistance across sensor terminals using a digital multimeter. At 68°F (20°C), resistance should read approximately 2,400-2,700 ohms. Check circuit voltage at the harness connector with ignition on: you should see 5 volts reference voltage from the ECM.

Step 3: Harness and Ground Verification Using a multimeter, test continuity between the sensor connector and ECM pins (consult Hyundai service manual PIN diagrams). Verify the ground circuit has less than 0.5 ohms resistance to engine block. For used machines, remove heat shrink or electrical tape to inspect hidden wire breaks.

Step 4: Component Replacement Replace the ECT sensor (Hyundai part #39240-4X700) if readings are out of specification. If the sensor tests correctly, inspect the ECM connector for terminal corrosion and repair/replace the harness as needed.

Disclaimer: This guide provides general troubleshooting information. Always consult the Hyundai R335LC-9T service manual and consider professional diagnostic services for complex electrical issues, especially when dealing with high-value used equipment.