What is Komatsu PC120LC-6 Fault Code S-15?

Komatsu PC120LC-6 Fault Code S-15 indicates an abnormality in the engine controller (pump controller) communication system — specifically, a disconnection or signal failure detected on the serial data communication line between the pump controller and the monitor panel.

This code is tied to the Controller Area Network (CAN) or serial communication bus that allows the machine's ECM (Engine Control Module), pump controller, and monitor display unit to exchange real-time operational data. When this communication link is disrupted, the machine loses its ability to coordinate hydraulic output demand with engine load management — a critical function for efficient and safe excavator operation.

On an aging machine like the PC120LC-6, this fault is taken seriously because unresolved communication failures can cascade into hydraulic derate conditions or unpredictable power delivery.

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Common Symptoms

When Fault Code S-15 is active on the PC120LC-6, operators typically report:

• Warning light illumination on the monitor panel, often accompanied by an audible alert buzzer
• Reduced hydraulic response or sluggish boom/arm/bucket movements due to pump output being limited by the controller's default safe mode
• Monitor display anomalies, including frozen readings, blank screens, or failure to display engine RPM and hydraulic pressure data
• Engine operating in "limp mode" — the machine may start and idle but refuse to accept full throttle commands
• Intermittent fault behavior, where the code appears and clears with vibration or temperature changes (a strong indicator of a wiring issue)

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Potential Causes

The following are the most common root causes for S-15 on used PC120LC-6 machines:

• Damaged or corroded serial communication harness — particularly in the main chassis harness running between the pump controller and monitor unit
• Faulty pump controller unit — internal communication chip failure, common on high-hour machines
• Loose or corroded connector pins at the CN-EP1 or CN-EP2 connector blocks on the pump controller housing
• Monitor panel unit failure — the display module itself losing its ability to receive serial data
• Power supply irregularities — low or unstable voltage to the pump controller (below 20V) causing communication dropout
• Ground circuit deterioration — corroded chassis ground points causing signal interference on the data line

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How to Troubleshoot and Fix Code S-15

Step 1 — Inspect the Communication Harness (Start Here on Used Machines)

Begin with a thorough visual inspection of the wiring harness routing from the pump controller to the monitor panel. On used PC120LC-6 units, pay special attention to areas where the harness passes through bulkheads, near hydraulic lines, or along the swing frame — these are known friction and abrasion points. Look for chafed insulation, pinched wires, or heat damage. Corrosion inside connector housings is extremely common on older machines; clean all contacts with electrical contact cleaner and inspect for bent or recessed pins.

Step 2 — Verify Power Supply and Ground to the Pump Controller

Using a digital multimeter, measure the supply voltage at the pump controller's main power connector with the key in the ON position. The specification should read 20–30V DC. Measure resistance on the ground circuit back to the battery negative; it should read near 0 Ω. A reading above 0.5 Ω indicates a deteriorated ground — clean and re-torque all chassis ground connections.

Step 3 — Check Serial Communication Line Continuity

With the key OFF and both controllers unplugged, use your multimeter in continuity/resistance mode to test the serial communication wire (typically a twisted pair or single data wire depending on harness type) between the pump controller connector and the monitor connector. Resistance should be near zero for a healthy circuit. Infinite resistance confirms an open circuit — trace and repair the break before replacing any components.

Step 4 — Use Komatsu KOMTRAX or EMMS Diagnostic Software

Connect Komatsu's PC Serviceability Tool (PC-SA Tool) or access the EMMS (Engine/Machine Monitoring System) data logger via the machine's diagnostic port. Pull the full fault history to determine whether S-15 is appearing alongside other controller codes, which helps isolate whether the failure is in the pump controller, monitor, or purely the communication line.

Step 5 — Component Substitution (Last Resort)

Only after confirming harness integrity and power supply should you consider replacing the pump controller unit or monitor panel. Unnecessary parts replacement is the most common and costly mistake on used excavator repairs. Always source replacement controllers with matching part numbers and confirm software compatibility before installation.

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Disclaimer: The diagnostic procedures outlined above are provided for informational purposes and are intended to assist qualified heavy diesel mechanics and equipment professionals. Always consult the official Komatsu PC120LC-6 Shop Manual and adhere to all applicable safety protocols before performing electrical or hydraulic system work. If you are unsure of any procedure, engage a Komatsu-authorized service technician.