Komatsu PC300-8M0 Fault Code 879BKB: Complete Diagnostic Guide

What is Komatsu PC300-8M0 Fault Code 879BKB?

Fault code 879BKB on the Komatsu PC300-8M0 indicates a malfunction in the engine coolant temperature sensor circuit, specifically detecting an abnormally high voltage signal or open circuit condition.

This diagnostic trouble code is generated by the Engine Control Module (ECM) when the coolant temperature sensor signal falls outside the manufacturer's specified parameters. The sensor, typically a negative temperature coefficient (NTC) thermistor, provides critical data for fuel injection timing, cold-start enrichment, and cooling fan activation.

On the PC300-8M0's SAA6D114E-5 engine, accurate coolant temperature readings are essential for optimal combustion efficiency and emissions compliance. Ignoring this fault can lead to improper fuel metering, increased fuel consumption, and potential engine damage from undetected overheating conditions.

Common Symptoms

• Check Engine Light (CEL) illuminated on the monitor panel
• Engine running in derate mode with reduced power output
• Cooling fans operating continuously or not engaging at all
• Erratic idle behavior, particularly during cold starts
• Inaccurate temperature readings on the operator display (showing maximum or minimum values)

Potential Causes

The most frequent causes of code 879BKB include:

• Coolant temperature sensor failure due to thermal cycling degradation
• Open circuit in the sensor wiring harness from chafing or rodent damage
• Corroded connector pins at the sensor or ECM connection points
• Damaged wiring insulation near exhaust manifolds or hot engine components
• ECM ground circuit issues affecting sensor signal interpretation

On used PC300-8M0 units, pay particular attention to harness routing near the turbocharger and exhaust system, where heat exposure accelerates wire deterioration.

How to Troubleshoot and Fix Code 879BKB

Step 1: Visual Inspection Locate the coolant temperature sensor on the engine block. Inspect the connector for corrosion, moisture intrusion, or bent pins. Examine the wiring harness for abrasion, especially where it contacts brackets or hot surfaces.

Step 2: Sensor Testing Using a digital multimeter, measure the sensor's resistance. At 20°C (68°F), expect approximately 2.5-3.0 kΩ. At operating temperature (80°C/176°F), resistance should drop to 300-350Ω. Values outside these ranges indicate sensor replacement is necessary.

Step 3: Circuit Verification With the sensor disconnected, use Komatsu KOMTRAX Plus or compatible diagnostic software to verify 5V reference voltage at the connector. Check continuity between the sensor connector and ECM terminals to identify open circuits.

Step 4: Connector Rehabilitation For used excavators, clean all connector pins with electrical contact cleaner and apply dielectric grease before reassembly. Replace any terminals showing green oxidation.

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Disclaimer: This guide is for informational purposes only. Always consult Komatsu service documentation and qualified technicians for definitive diagnosis and repairs.