Hitachi Excavator Fault Code P0108: Complete Diagnostic Guide

What is Hitachi Fault Code P0108?

Fault Code P0108 indicates a Manifold Absolute Pressure (MAP) sensor circuit high voltage condition. This diagnostic trouble code (DTC) triggers when the Engine Control Module (ECM) detects a voltage signal from the MAP sensor that exceeds the manufacturer's specified maximum threshold, typically above 4.5 volts.

The MAP sensor is critical for Hitachi excavators because it monitors intake manifold pressure to calculate proper fuel injection timing and air-fuel ratios. When this sensor reports abnormally high voltage, the ECM cannot accurately determine engine load, leading to compromised performance, excessive fuel consumption, and potential damage to emission control systems. For used excavators operating in dusty or high-vibration environments, this code often signals sensor degradation or wiring issues specific to aging machines.

Common Symptoms

When P0108 is active on your Hitachi excavator, you may experience:

• Check Engine Light (CEL) or malfunction indicator lamp illuminated on the dashboard
• Engine derate mode – noticeable power loss, especially under heavy load or during digging operations
• Rough idle or unstable engine RPM when at operating temperature
• Black smoke from the exhaust due to incorrect fuel delivery calculations
• Hard starting or extended cranking time, particularly in cold conditions

Potential Causes

The most common technical causes for P0108 on used Hitachi excavators include:

• Failed MAP sensor – internal component failure or contamination from oil vapor/carbon buildup
• Damaged wiring harness – particularly at known rub points near the engine firewall or along the valve cover where vibration causes insulation wear
• Corroded or loose connector pins at the MAP sensor plug (three-wire connector), common in machines exposed to moisture or mud
• Shorted signal wire to battery voltage anywhere along the harness route
• Faulty ECM (rare but possible in high-hour used machines with electrical system history)
• Vacuum leak affecting manifold pressure reading (less common cause for high voltage codes)

How to Troubleshoot and Fix Code P0108

Step 1: Visual Inspection Begin by locating the MAP sensor (typically mounted on the intake manifold). Inspect the three-wire connector for corrosion, bent pins, or moisture intrusion. On used excavators, check the entire wiring harness for chafing against metal surfaces, particularly where it passes near moving components or sharp edges.

Step 2: Electrical Testing Using a digital multimeter (DMM), perform a voltage test with the ignition on, engine off. Backprobe the signal wire (typically the center pin) – it should read between 4.5-5.0 volts with the sensor disconnected. If voltage remains high with the sensor unplugged, you have a wiring short to power. Check resistance between the signal wire and ground; it should read infinite (open circuit).

Step 3: Sensor Testing With the sensor connected and engine running at idle, measure the signal voltage – it should be approximately 1.0-1.5 volts. Rev the engine; voltage should increase smoothly. Erratic readings indicate a failing sensor. For used machines, also apply a vacuum hand pump directly to the sensor port while monitoring voltage to verify proper response.

Step 4: Component Replacement If testing confirms sensor failure, replace the MAP sensor with an OEM or high-quality equivalent part. Before installation, clean the mounting port and apply dielectric grease to connector terminals to prevent future corrosion. After replacement, clear codes using Hitachi diagnostic software (Dr.EX) or compatible scan tool and perform a test cycle under load.

Professional Tip for Used Equipment: Always inspect harness routing after repairs. Secure any loose wiring away from heat sources and moving parts using OEM-style clips to prevent recurrence.

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Disclaimer: This guide provides general diagnostic information. Always consult your Hitachi service manual and consider professional assistance for complex electrical diagnostics, especially on high-hour used equipment where multiple system issues may coexist.