Kobelco SK135SRLC-2 Fault Code R044: Complete Diagnostic Guide

What is Kobelco SK135SRLC-2 Fault Code R044?

Fault Code R044 indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading beyond the acceptable range. This code is triggered when the Electronic Control Module (ECM) detects voltage signals from the coolant temperature sensor that fall outside the manufacturer's specified parameters (typically 0.2-4.8V), or when the sensor reports temperatures inconsistent with actual engine operating conditions.

The coolant temperature sensor is critical for the SK135SRLC-2's performance because it directly influences fuel injection timing, idle speed control, and engine protection protocols. When this sensor fails or provides incorrect data, the ECM cannot properly manage engine temperature, potentially leading to overheating, poor fuel economy, or inadequate cold-start performance. For used excavators, this fault often results from sensor degradation or wiring issues common in machines with high operating hours.

Common Symptoms

• Engine warning lamp illuminated on the instrument panel (amber or red depending on severity)
• Hard starting or extended cranking time, especially during cold weather conditions
• Rough idle or unstable engine RPM when the machine is first started
• Black smoke from exhaust due to improper fuel mixture calibration
• Engine derate mode activated, limiting maximum RPM and reducing hydraulic performance to protect the engine from potential damage

Potential Causes

The most common technical causes for Code R044 on used SK135SRLC-2 excavators include:

• Faulty coolant temperature sensor due to internal element deterioration (common after 5,000+ operating hours)
• Corroded or loose electrical connectors at the sensor harness connection point
• Damaged wiring harness caused by rubbing against engine components or heat damage near the exhaust manifold
• Low coolant level causing the sensor to read air temperature instead of coolant temperature
• Contaminated coolant creating resistance deposits on the sensor probe
• ECM internal fault (rare, but possible in machines exposed to moisture intrusion)

How to Troubleshoot and Fix Code R044

Step 1: Visual Inspection Begin by checking the coolant level in the radiator and overflow reservoir. Low coolant is a frequent cause in used machines with minor leaks. Inspect the sensor connector located on the engine block (typically near the thermostat housing on the SK135SRLC-2). Look for corrosion, moisture, or damaged pins. Clean connectors with electrical contact cleaner and ensure tight connections.

Step 2: Sensor Circuit Testing Using a digital multimeter, measure the resistance across the coolant temperature sensor terminals with the engine cold. At approximately 20°C (68°F), resistance should read between 2,000-3,000 ohms. As temperature increases to 80°C (176°F), resistance should drop to approximately 200-400 ohms. Values significantly outside this range indicate a failed sensor requiring replacement (Kobelco part number typically YN20M00036F1).

Step 3: Wiring Harness Verification Disconnect the sensor and measure continuity from the ECM connector to the sensor harness connector. Check for shorts to ground or open circuits. On used excavators, inspect harness routing near the engine mount brackets and coolant hoses where friction wear commonly occurs. Repair or replace damaged sections using heat-shrink connectors and proper abrasion-resistant sleeving.

Step 4: Coolant System Check Verify coolant mixture ratio (50/50 antifreeze/water) using a refractometer. Contaminated or incorrect coolant can cause sensor fouling. If all electrical tests pass but the code persists, consider ECM reprogramming or updating using Kobelco diagnostic software (KEMS or compatible diagnostic tool).

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Disclaimer: This guide provides general diagnostic information for experienced technicians. Always consult the official Kobelco service manual for your specific machine's serial number range. If you lack proper diagnostic equipment or experience with electrical systems, consult a certified Kobelco dealer or qualified heavy equipment technician to prevent misdiagnosis or component damage.