What is Komatsu PC120LC-6 Fault Code S-2?

Fault Code S-2 on the Komatsu PC120LC-6 indicates a malfunction in the pilot pressure sensor circuit. This sensor monitors the hydraulic pilot pressure that controls auxiliary hydraulic functions and implements precise joystick operations. The Electronic Control Module (ECM) uses this data to regulate hydraulic flow and optimize fuel efficiency during combined operations.

This code is critical for the PC120LC-6 because the pilot pressure system directly affects the proportional control of attachments and swing functions. When the pilot pressure sensor fails or provides erratic readings, the machine cannot accurately modulate hydraulic output, leading to jerky movements or reduced implement responsiveness. On used excavators, degraded sensor performance can mask underlying hydraulic system issues that accelerate component wear.

Common Symptoms

• Erratic or unresponsive implement control, particularly during fine positioning work where proportional control is essential
• Intermittent warning light on the monitor panel, often triggered by vibration or temperature changes
• Reduced auxiliary hydraulic flow, making attachment operations sluggish or inconsistent
• Abnormal joystick feedback, where input doesn't match hydraulic response proportionally
• Engine derate mode in some cases, though less common with S-2 specifically

Potential Causes

The most common causes for S-2 on used PC120LC-6 excavators include:

• Pilot pressure sensor failure due to internal diaphragm deterioration or contamination from dirty hydraulic oil
• Corroded or loose electrical connectors at the sensor mounting location, especially on machines operating in wet or marine environments
• Damaged wiring harness near the pilot valve assembly where cable routing creates friction points against the cab frame
• Low pilot circuit pressure caused by worn pilot pump or clogged pilot filters, generating readings outside normal parameters
• ECM calibration drift on older machines with high operational hours

How to Troubleshoot and Fix Code S-2

Step 1: Visual Inspection
Check the pilot pressure sensor connector located near the pilot valve block for corrosion, oil contamination, or damaged pins. Inspect the wiring harness routing for chafing points where the loom contacts metal edges—common failure points on the PC120LC-6 near the cab mount.

Step 2: Electrical Testing
Using a digital multimeter, measure sensor supply voltage (should be 5V ±0.25V) with the key on, engine off. Check signal wire resistance to ground (should read open circuit). Verify ground circuit continuity with less than 5 ohms resistance.

Step 3: Sensor Output Verification
With the engine running, monitor sensor output voltage using Komatsu diagnostic software (KOMTRAX or PC Service Tool). Normal range is 0.5-4.5V proportional to pilot pressure. If readings are erratic or frozen, replace the pilot pressure sensor.

Step 4: Hydraulic System Check
On used excavators, verify actual pilot circuit pressure using a mechanical pressure gauge (normal range: 3.5-4.0 MPa). Replace the pilot filter and check for internal pump wear if pressure is low.

Disclaimer: This guide provides general troubleshooting steps. Consult a certified Komatsu technician or authorized service manual for model-specific procedures and safety protocols.