Kobelco SK135SRLC-2 Fault Code A025: Complete Diagnostic Guide

What is Kobelco SK135SRLC-2 Fault Code A025?

Fault Code A025 indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading detected by the ECM (Engine Control Module). This code specifically monitors the thermistor-type sensor that measures engine coolant temperature and transmits data to the machine's electronic control system.

The coolant temperature sensor plays a critical role in the SK135SRLC-2's Isuzu engine management system. It directly influences fuel injection timing, idle speed control, and engine protection protocols. When this sensor fails or sends erratic signals, the ECM cannot properly regulate engine performance, potentially leading to overheating, poor fuel economy, or engine damage. For used excavators, this code often appears due to age-related sensor degradation or harness deterioration common in machines with 5,000+ operating hours.

Common Symptoms

When Code A025 is active, operators typically experience:

• Engine warning light illuminated on the instrument panel, often accompanied by a buzzer alert
• Difficulty starting the engine, especially during cold weather conditions due to improper fuel enrichment
• Rough idle or hunting RPM as the ECM struggles to maintain proper engine speed without accurate temperature data
• Engine running hotter than normal or entering derate/limp mode to protect against perceived overheating
• Black smoke from exhaust indicating overly rich fuel mixture from compensation strategies

Potential Causes

The most common technical reasons for Code A025 in used SK135SRLC-2 excavators include:

• Failed coolant temperature sensor (NTC thermistor failure is typical after 6,000-8,000 hours)
• Damaged or corroded wiring harness connections at the sensor connector or ECM pins
• Shorted or open sensor circuit caused by harness chafing against engine components or frame rails
• Contaminated sensor tip from improper coolant mixture or accumulated debris in coolant passages
• ECM internal fault (rare, but possible in high-hour machines with moisture intrusion)
• Incorrect coolant level causing intermittent sensor exposure and erratic readings

How to Troubleshoot and Fix Code A025

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the engine block near the thermostat housing. Check the two-wire connector for corrosion, bent pins, or moisture intrusion—common issues in used excavators. Examine the entire wiring harness from sensor to ECM for rubbing points, particularly where it passes near the engine mount and hydraulic lines.

Step 2: Sensor Resistance Testing Disconnect the sensor and use a digital multimeter to measure resistance across the sensor terminals. At 68°F (20°C), resistance should read approximately 2,000-3,000 ohms. Compare readings against Kobelco specifications. For accurate testing on used machines, heat the sensor in water while monitoring resistance—it should decrease smoothly as temperature rises. Erratic readings indicate sensor failure.

Step 3: Circuit Voltage Check With the sensor disconnected and ignition ON, measure reference voltage at the harness connector. You should see approximately 5 volts on the signal wire. Check for proper ground continuity (less than 1 ohm resistance to chassis ground). If voltage is absent or incorrect, trace the harness for breaks or shorts.

Step 4: Component Replacement Replace the coolant temperature sensor using OEM or equivalent parts (verify NTC resistance curve matches original specifications). Apply dielectric grease to connector terminals before reconnection—critical for preventing corrosion in used equipment. Clear codes using Kobelco diagnostic software or a compatible scan tool, then monitor for recurrence during a complete heat cycle.

Disclaimer: This guide provides general diagnostic information. Always consult the Kobelco SK135SRLC-2 service manual for exact specifications and procedures. If unfamiliar with electrical diagnostics or engine systems, seek assistance from a qualified heavy equipment technician to prevent further damage.