Hyundai R335LC-9T Fault Code 585: Complete Diagnostic Guide

What is Hyundai R335LC-9T Fault Code 585?

Fault Code 585 on the Hyundai R335LC-9T indicates a malfunction in the Engine Coolant Temperature (ECT) Sensor Circuit, specifically detecting an abnormally high voltage signal or open circuit condition. This code triggers when the Engine Control Module (ECM) receives a voltage reading outside the acceptable range (typically above 4.8V) from the coolant temperature sensor.

The ECT sensor is critical for the R335LC-9T's Cummins QSB6.7 engine management system. It directly influences fuel injection timing, air-fuel mixture ratios, and diesel exhaust fluid (DEF) dosing rates. When this sensor circuit fails, the ECM cannot accurately monitor engine operating temperature, potentially causing inefficient combustion, increased emissions, and premature component wear in used machines with higher operating hours.

Common Symptoms

Operators experiencing Code 585 typically encounter:

• Check Engine Light illuminated on the instrument cluster with possible high coolant temperature warning
• Engine derate mode activated, limiting maximum RPM to 1800 or reducing power output by 25-40%
• Hard starting conditions, particularly in cold weather, as the ECM defaults to cold-start enrichment parameters
• Rough idle or excessive white smoke during warm-up due to incorrect fuel mapping
• DEF system warnings may appear simultaneously, as coolant temperature affects aftertreatment calibration

Potential Causes

The most common technical failures triggering Code 585 on used R335LC-9T excavators include:

• Open circuit in the ECT sensor wiring harness, particularly at known rub points near the engine block's left-side mounting brackets where vibration causes insulation wear
• Failed ECT sensor with internal resistance breakdown (common after 6,000+ operating hours)
• Corroded or loose connector terminals at the sensor plug or ECM harness connection (especially in machines operating in coastal or high-humidity environments)
• Damaged ECM pin connections at terminals 23/24 of the 120-pin engine harness connector
• Coolant contamination causing sensor element deterioration in machines with poor maintenance history

How to Troubleshoot and Fix Code 585

Step 1: Visual Inspection Perform a thorough inspection of the ECT sensor located on the thermostat housing near the front of the engine block. Check for coolant leaks, corrosion on the two-pin connector, and physical damage to the wiring harness along its routing path toward the ECM.

Step 2: Electrical Testing Disconnect the ECT sensor connector and measure resistance across the sensor terminals using a digital multimeter. At 68°F (20°C), resistance should read approximately 2,400-2,700 ohms. Check for open circuit (infinite resistance) or values outside specification. Also verify supply voltage at the harness connector (key on, engine off) reads approximately 5.0V.

Step 3: Harness Continuity Check With the sensor disconnected, test continuity and resistance between the sensor connector and ECM terminals 23 (signal) and 24 (ground). Resistance should be less than 5 ohms. For used excavators, carefully inspect the harness at frame contact points and where it passes near the hydraulic pump—these are common failure locations.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Hyundai or OEM-equivalent ECT sensor (part reference: 3969597). If wiring is damaged, repair using weatherproof heat-shrink solder connections rather than crimp connectors. After replacement, clear codes using Hyundai Hi-MATE diagnostic software or compatible scan tool and verify coolant temperature readings match actual engine temperature.

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Disclaimer: This guide provides general diagnostic information for Code 585. Complex electrical issues or internal ECM failures require diagnosis by certified Hyundai construction equipment technicians with manufacturer-specific diagnostic tools. Always consult the official service manual and follow proper lockout/tagout procedures before performing repairs.