Komatsu PC120LC-6 Fault Code E101: Complete Diagnostic Guide

What is Komatsu PC120LC-6 Fault Code E101?

Fault Code E101 on the Komatsu PC120LC-6 indicates a malfunction in the engine controller (ECM) power supply circuit or an internal ECM failure. This code specifically flags an abnormality in the primary voltage feed to the engine control module, which serves as the central brain for fuel injection timing, engine speed regulation, and emissions control.

On the PC120LC-6 equipped with the SAA4D95LE-3 engine, the ECM requires stable 12V or 24V input depending on configuration. When voltage drops below threshold parameters or experiences excessive fluctuation, the system logs E101. This fault is critical because an unstable ECM power supply can cause erratic engine behavior, improper fuel delivery, and potential long-term damage to electronic components.

Common Symptoms

• Engine fails to start or cranks without firing
• Intermittent engine shutdown during operation
• Warning lamp illumination on the monitor panel
• Erratic idle speed or hunting RPM
• Loss of communication with diagnostic equipment (VHMS or Komatsu DST)

Potential Causes

The most frequent causes of E101 on used PC120LC-6 units include:

• Corroded battery terminals or loose connections at the battery disconnect switch
• Damaged wiring harness near the swing motor junction (a known rub point on this model)
• Failed main relay in the electrical panel
• Internal ECM failure due to moisture intrusion or age-related component degradation
• Blown fuse in the ECM power circuit (typically 10A or 15A)
• Poor ground connection at the engine block mounting point

How to Troubleshoot and Fix Code E101

Step 1: Verify Battery and Primary Power Using a multimeter, check battery voltage at the terminals. Confirm readings between 24V–28V (or 12V–14V for 12V systems). Inspect terminals for corrosion. On used machines, examine the battery disconnect switch contacts for wear or pitting.

Step 2: Inspect Wiring Harness and Connectors Trace the ECM power harness from the battery through the main fuse box. Pay particular attention to areas near the hydraulic pump and swing frame pivot—harness chafing is common here. Check the ECM connector pins for bent terminals, green corrosion, or moisture residue. Clean with electrical contact cleaner.

Step 3: Test ECM Power Supply at Source With the key in the ON position, measure voltage at the ECM connector pins (refer to the service manual for pin identification). Voltage should match battery voltage within 0.5V. If voltage is absent, test the main relay for proper operation using Komatsu DST-PC or a relay tester.

Step 4: Evaluate ECM Condition If power supply checks pass but E101 persists, the ECM may require replacement. On older machines, confirm no previous water damage or improper jump-starting occurred, as these commonly cause internal ECM failure.

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Disclaimer: This guide provides general technical information. Always consult the official Komatsu shop manual and consider professional diagnosis before replacing major components.