What is Hyundai R275LVS Fault Code 351?

Fault Code 351 on the Hyundai R275LVS excavator indicates a malfunction in the Engine Coolant Temperature (ECT) Sensor Circuit - specifically a voltage signal that is higher than the normal operating range. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormally high voltage reading from the coolant temperature sensor, typically above 4.5-4.8 volts depending on calibration.

The ECT sensor is critical for the R275LVS because it directly influences fuel injection timing, engine idle speed, and cooling fan operation. When this sensor circuit malfunctions, the ECM cannot accurately monitor engine temperature, potentially leading to improper fuel delivery, overheating risk, or unnecessary cooling system activation that reduces operational efficiency.

Common Symptoms

• Check Engine Light (CEL) or malfunction indicator lamp illuminates on the dashboard
• Engine runs rough during cold start or fails to reach normal operating temperature quickly
• Cooling fan runs continuously at high speed, even when engine is cold
• Reduced engine power or derate mode activation to protect engine components
• Abnormally high temperature gauge readings or erratic temperature display fluctuations

Potential Causes

The most frequent causes of Code 351 on used R275LVS excavators include:

• Open circuit in the ECT sensor wiring harness, often caused by wire breakage near engine mounting points where vibration stress is highest
• Failed ECT sensor with internal circuit degradation (common after 5,000+ operating hours)
• Corroded or damaged electrical connectors at the sensor or ECM, particularly from coolant seepage or moisture intrusion
• Chafed wiring insulation creating intermittent short-to-power conditions along the engine harness routing path
• Faulty ECM ground connections causing voltage reference errors (less common but documented on older units)

How to Troubleshoot and Fix Code 351

Step 1: Visual Inspection Inspect the ECT sensor connector located near the thermostat housing on the engine block. Check for corrosion, bent pins, or coolant contamination. On used excavators, examine the entire sensor harness route for wear points where the wiring contacts engine components or frame members.

Step 2: Electrical Testing Using a digital multimeter, disconnect the ECT sensor and measure resistance across the sensor terminals. At 68°F (20°C), resistance should read approximately 2,000-3,000 ohms (verify exact specification in service manual). Next, check for continuity between the sensor connector and ECM pins - infinite resistance indicates an open circuit requiring harness repair.

Step 3: Voltage Reference Check With ignition ON and sensor disconnected, measure voltage at the signal wire (typically the sensor harness connector). You should see approximately 5 volts (ECM reference voltage). If voltage reads battery voltage (12V+), suspect a short-to-power condition in the harness.

Step 4: Sensor Replacement or Repair If sensor resistance is out of specification or physical damage is evident, replace the ECT sensor using OEM or equivalent quality parts. For used machines, always apply dielectric grease to connectors during reinstallation to prevent future corrosion issues.

Step 5: Clear Codes and Verify After repairs, clear fault codes using Hyundai diagnostic software (Hi-MATE or equivalent tool) and operate the machine through a complete warm-up cycle to confirm the repair.

---

Disclaimer: This guide provides general troubleshooting information. Always consult the official Hyundai service manual for your specific serial number and consider professional diagnostic assistance for complex electrical issues or if you're unfamiliar with high-voltage systems.