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

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

Fault Code 879EMC indicates an Engine Monitoring Computer (EMC) communication error or abnormal signal detected in the machine's electronic control system. This code specifically relates to a breakdown in data transmission between the Engine Control Module (ECM) and other critical monitoring systems on the Komatsu PC300-8M0 excavator.

The 879EMC code affects the CAN bus communication network, which coordinates engine performance, hydraulic functions, and operator display information. When this communication fails, the machine cannot properly regulate fuel injection timing, monitor emissions systems, or provide accurate diagnostic feedback. For the PC300-8M0's Komatsu SAA6D114E engine, maintaining reliable ECM communication is essential for optimal fuel efficiency and meeting Tier 3/Tier 4 emissions standards.

Common Symptoms

When fault code 879EMC is active, operators typically experience:

• Check Engine light or malfunction indicator illuminated on the monitor panel
• Intermittent loss of engine data on the digital display (RPM, coolant temperature, fuel level showing incorrect readings)
• Engine derate mode activation, limiting power output to 70-80% capacity
• Erratic throttle response or unexpected engine speed fluctuations
• Inability to access diagnostic information through the monitor screen

Potential Causes

The most common technical reasons for 879EMC on used PC300-8M0 excavators include:

• Corroded or loose connector pins at the ECM harness (C201 connector particularly vulnerable)
• CAN bus wiring damage from harness rubbing against frame rails near the battery box area
• Failed EMC relay or intermittent power supply to the monitoring computer
• ECM internal failure due to moisture intrusion or voltage spikes (common in machines with 8,000+ operating hours)
• Grounding issues at chassis ground points G101 or G102, often corroded on older units
• Aftermarket component interference from non-OEM radios or tracking devices disrupting CAN communication

How to Troubleshoot and Fix Code 879EMC

Step 1: Visual Inspection Begin by inspecting the main ECM connector (located behind the operator cab, left side). Check for corrosion, bent pins, or moisture in the connector housing. On used excavators, examine the entire engine harness for chafing points where it passes through the frame—this is a known wear point on PC300-8M0 models.

Step 2: Electrical Testing Using a digital multimeter, verify power supply to the ECM. Check for 12-24V DC at pin 1 (power) and continuity to ground at pin 32. Test CAN-High and CAN-Low resistance between ECM and monitor (should read 60 ohms across the network). Komatsu's KOMTRAX diagnostic software can identify specific communication dropout patterns.

Step 3: Component Isolation Disconnect aftermarket accessories and retest. Clean all chassis ground points with a wire brush and apply dielectric grease. If issues persist, swap the EMC relay (located in the main fuse box) with an identical relay to rule out relay failure.

Step 4: Advanced Diagnosis For persistent codes, perform a CAN bus oscilloscope test to identify signal integrity issues. On high-hour used machines, ECM replacement may be necessary—always use genuine Komatsu parts (part #600-468-1200) to ensure compatibility.

---

Disclaimer: This guide provides general diagnostic information. Complex electrical issues should be diagnosed by certified Komatsu technicians with proper diagnostic equipment. Always consult the official service manual for your specific machine serial number.