What is Komatsu PC120-6 Fault Code E308?

Komatsu PC120-6 Fault Code E308 indicates a malfunction in the hydraulic pump discharge pressure sensor circuit. This code specifically signals that the Electronic Control Module (ECM) has detected an abnormal voltage reading from the pressure sensor monitoring the main hydraulic pump's output pressure.

The pressure sensor plays a critical role in the PC120-6's load-sensing hydraulic system, continuously feeding pressure data to the ECM to optimize pump flow and prevent system overload. When E308 triggers, the machine may enter a protective derate mode to prevent potential hydraulic system damage. For used excavators, this code often appears due to sensor degradation, connector corrosion, or wiring harness damage from years of vibration and environmental exposure.

Common Symptoms

• Warning lamp illuminated on the instrument panel with reduced hydraulic performance
• Sluggish hydraulic response across all functions, particularly during combined operations
• Intermittent power loss where the machine suddenly loses digging force or swing speed
• Engine derating to lower RPM ranges, preventing full throttle operation
• Erratic pressure readings visible on machines equipped with hydraulic pressure gauges

Potential Causes

The most frequent technical causes for E308 on used PC120-6 excavators include:

• Pressure sensor failure due to internal diaphragm fatigue or contaminated hydraulic oil entering the sensor body
• Corroded or loose connector pins at the sensor plug, especially common in high-moisture operating environments
• Damaged wiring harness where the sensor cable runs along the pump housing, subject to heat and vibration wear
• ECM voltage supply issues providing incorrect reference voltage (typically 5V) to the sensor circuit
• Hydraulic oil contamination causing sensor port blockage and inaccurate pressure readings
• Previous repair attempts with incorrect sensor torque specifications causing internal sensor damage

How to Troubleshoot and Fix Code E308

Step 1: Visual Inspection Begin by locating the hydraulic pump discharge pressure sensor mounted on the main pump assembly. Inspect the electrical connector for corrosion, oil contamination, or pushed-back pins. On used excavators, check for harness chafing where the wire bundle contacts the pump housing or nearby hydraulic lines.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure the ECM supply voltage at the harness connector—should read approximately 5 volts DC with ignition on. Check sensor resistance (typically 200-400 ohms at room temperature) and compare against Komatsu specifications. Inspect for short circuits to ground in the wiring.

Step 3: Sensor and Circuit Verification If voltage and resistance are correct, reconnect the sensor and use Komatsu diagnostic software to monitor real-time pressure values during operation. Compare readings against actual hydraulic system pressure (use a mechanical gauge at a test port). Replace the pressure sensor if readings deviate significantly or fluctuate erratically.

Step 4: Critical Used Equipment Checks Before replacing components, thoroughly clean all connector terminals with electrical contact cleaner and apply dielectric grease. Examine the entire sensor harness for insulation cracks or previous field repairs using improper connectors.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual and consider professional technician assistance for complex electrical diagnostics or hydraulic system repairs.