What is Komatsu PC120LC-6 Fault Code H-25?

Fault Code H-25 on the Komatsu PC120LC-6 indicates a hydraulic pump solenoid proportional valve circuit malfunction. This code specifically signals an electrical abnormality in the proportional control circuit that regulates the variable displacement hydraulic pump's output flow.

The controller monitors current flow to the pump control solenoid, which adjusts pump displacement based on hydraulic demand. When the ECM detects voltage or current values outside the normal operating range (typically 12-24VDC with specific milliamp draw), it triggers H-25. This is critical because the proportional valve directly controls hydraulic power delivery—affecting digging force, swing speed, and overall machine responsiveness. On used excavators, this fault often indicates age-related electrical degradation rather than catastrophic pump failure.

Common Symptoms

• Yellow warning light illuminated on the instrument panel with reduced hydraulic power
• Sluggish boom, arm, or bucket movements with noticeably slower cycle times
• Erratic hydraulic response where functions work intermittently or require multiple control inputs
• Engine operates normally but machine performance is significantly derated to prevent pump damage
• Pump produces unusual whining or cavitation sounds when operating under load

Potential Causes

The most frequent causes on older PC120LC-6 machines include:

• Damaged wiring harness between the ECM and pump solenoid—common rub points exist where the harness passes near the swing motor housing and along the pump mounting area
• Corroded or moisture-contaminated connectors at the solenoid plug (4-pin Deutsch connector on this model)
• Failed proportional solenoid coil with resistance outside specification (normal resistance: 3-6 ohms at 20°C)
• Shorted or open circuits in the control wiring due to insulation breakdown from heat exposure near the engine
• ECM internal driver failure (less common but occurs in high-hour machines)

How to Troubleshoot and Fix Code H-25

Step 1: Visual Inspection
Disconnect the battery and inspect the pump solenoid connector and entire wiring harness for physical damage, corrosion, or oil contamination. On used excavators, carefully check where harnesses contact metal edges—years of vibration create wear points. Clean all connector pins with electrical contact cleaner.

Step 2: Resistance Testing
Using a digital multimeter, disconnect the solenoid connector and measure resistance across the solenoid terminals. Compare against specifications (typically 3-6 ohms). If readings are infinite (open circuit) or near zero (shorted), replace the proportional solenoid valve.

Step 3: Voltage and Continuity Checks
With ignition on (engine off), measure supply voltage at the harness-side connector—should read battery voltage. Check continuity between ECM pins and the solenoid connector. Repair or replace the harness if readings are inconsistent.

Step 4: Solenoid Replacement
If electrical tests pass but the code persists, replace the pump control solenoid assembly. Genuine Komatsu parts are recommended for proper current draw characteristics. After installation, use Komatsu diagnostic software (KOMTRAX or KDW) to clear codes and verify real-time solenoid current values.

Step 5: Professional Diagnosis
If H-25 returns after repairs, ECM reprogramming or replacement may be necessary—consult a Komatsu-certified technician.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu service manual and consider professional diagnosis for complex electrical issues on high-value equipment.