What is Komatsu PC128US-2 Fault Code E308?

Fault Code E308 on the Komatsu PC128US-2 indicates a malfunction in the Engine Oil Pressure Sensor Circuit. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormal voltage signal from the oil pressure sensor, typically indicating either an open circuit, short circuit, or sensor reading outside the normal operating range.

This code is critical for the PC128US-2 because proper oil pressure monitoring directly protects the engine from catastrophic damage. The Komatsu SAA4D95LE-3 engine in this model relies on accurate oil pressure readings to ensure adequate lubrication of internal components. When the ECM cannot verify proper oil pressure through sensor data, it may initiate derate modes or prevent machine operation to avoid engine seizure or bearing failure—a common and expensive problem in used excavators with sensor monitoring issues.

Common Symptoms

When fault code E308 is active on your PC128US-2, operators typically experience:

• Engine warning light or malfunction indicator illuminated on the dashboard display
• Engine derate or refusal to reach full RPM, limiting hydraulic performance and digging power
• Audible alarm or buzzer activation warning of oil pressure concerns
• Intermittent fault occurrence, especially during cold starts or when the machine is operating on inclines
• Loss of power during high-load operations such as digging or lifting heavy buckets

Potential Causes

The most common technical reasons for E308 in used PC128US-2 excavators include:

• Failed oil pressure sensor due to internal diaphragm deterioration or electrical contact corrosion (common after 5,000+ operating hours)
• Damaged or chafed wiring harness at known rub points near the engine firewall or along the frame rail where the harness routes toward the sensor
• Corroded or loose electrical connectors at the sensor plug or ECM interface, particularly in machines exposed to moisture or mud
• Low engine oil level or use of incorrect viscosity oil causing actual low pressure conditions
• Faulty ECM (rare, but possible in older machines with water intrusion into the controller compartment)
• Intermittent grounding issues in the sensor circuit due to worn mounting brackets or loose ground straps

How to Troubleshoot and Fix Code E308

Step 1: Visual Inspection and Oil Level Verification

Begin by checking the engine oil level using the dipstick. Ensure oil is at the proper level and that the correct SAE 10W-30 or 15W-40 viscosity oil (per Komatsu specifications) has been used. Inspect the oil pressure sensor location on the engine block—typically mounted near the oil filter on the SAA4D95LE-3 engine. Look for obvious signs of oil leaks, damaged connectors, or corroded terminals at the sensor plug.

Step 2: Electrical Circuit Testing

Using a digital multimeter, disconnect the oil pressure sensor and measure the sensor resistance. A functional sensor should read between 50-300 ohms (consult your specific service manual for exact values). Next, check for voltage supply at the sensor harness connector with the ignition on—you should see approximately 5 volts on the signal wire. Inspect the wiring harness from sensor to ECM for continuity, checking for opens or shorts to ground. Pay special attention to harness routing points where abrasion against frame components commonly occurs on used machines.

Step 3: Component Replacement and Verification

If sensor resistance is out of specification or wiring shows damage, replace the faulty component. For used excavators, always inspect and clean electrical connectors with electrical contact cleaner before reassembly—corrosion buildup is extremely common. After replacement, use Komatsu diagnostic software (KTCS or compatible scanner) to clear the fault code and monitor live oil pressure data during engine operation. Verify readings are within the normal range of 30-60 PSI at idle and 40-70 PSI at operating RPM. Test the machine under load to confirm the code does not return.

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Disclaimer: This guide provides general troubleshooting information for educational purposes. Always consult the official Komatsu service manual for your specific machine serial number and seek assistance from qualified heavy equipment technicians when performing diagnostics or repairs involving engine management systems.