What is Komatsu PC120LC-6 Fault Code S-6?

Fault Code S-6 on the Komatsu PC120LC-6 indicates a malfunction in the Engine Control Module (ECM) circuit, specifically related to the engine control system's communication or internal processor error. This diagnostic trouble code signals that the excavator's electronic control unit has detected an abnormality in its own operational parameters or data transmission pathways.

The S-6 code represents a critical system-level fault within the Komatsu SAA4D102E-2 engine's electronic management system. When this code appears, the ECM is reporting either internal computation errors, corrupted memory data, or compromised communication between control modules. This is particularly significant for the PC120LC-6 because the ECM governs fuel injection timing, engine speed regulation, and emission controls—all essential for optimal hydraulic pump performance and machine productivity.

Common Symptoms

• Warning lamp illuminated on the instrument cluster, often accompanied by reduced engine power or derating
• Engine may enter limp-home mode, limiting RPM to protect internal components
• Intermittent loss of throttle response or inconsistent engine performance during operation
• Hydraulic functions (swing, boom, bucket) operating sluggishly due to reduced engine output
• Potential engine stalling or difficulty starting, especially on older machines with accumulated ECM wear

Potential Causes

The S-6 fault code typically stems from several technical issues common in used excavators:

• Corroded or damaged ECM connectors exposed to moisture infiltration from damaged cab sealing
• Internal ECM capacitor failure or solder joint deterioration common in machines with 8,000+ operating hours
• Voltage irregularities from failing alternators or degraded main wiring harnesses (particularly the engine-to-frame ground strap)
• CAN bus communication errors caused by damaged shielding on control wiring near the turbocharger heat zone
• Battery voltage instability during cold starts, causing temporary ECM reset errors that trigger logged codes

How to Troubleshoot and Fix Code S-6

Step 1: Perform Initial Electrical Checks Using a digital multimeter, verify battery voltage is within 24-28V (engine running). Check the ECM power supply circuit at connector pins for proper voltage (typically 24V constant). Inspect all ground connections, especially the engine block ground strap for corrosion or looseness.

Step 2: Inspect ECM Harness and Connectors On used excavators, physically examine the main ECM connector (located behind the operator cab's right-side panel) for green corrosion, bent pins, or moisture intrusion. Check harness routing near the fuel injection pump where vibration-induced chafing commonly occurs on PC120LC-6 models.

Step 3: Test ECM Communication Connect Komatsu KOMTRAX diagnostic software or compatible scan tool. Monitor real-time data streams to verify the ECM is communicating properly with other modules. Check for additional stored codes that may indicate related failures.

Step 4: ECM Replacement Considerations If voltage tests pass but communication errors persist, the ECM internal circuitry may have failed. For used machines, consider professional ECM bench testing before replacement, as repair services can recover units at 40-60% cost savings versus new modules.

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Disclaimer: This guide provides general diagnostic direction. Always consult qualified diesel technicians familiar with Komatsu systems and reference the official PC120LC-6 service manual for complete procedures and specifications.