Kobelco SK200-8S Fault Code P0686: Complete Diagnostic Guide

What is Kobelco SK200-8S Fault Code P0686?

Fault Code P0686 indicates a malfunction in the main relay control circuit, specifically detecting an abnormal voltage condition in the ECM (Engine Control Module) power relay system. This code is triggered when the excavator's electronic control system identifies that the main relay—responsible for supplying power to critical engine control components—is not receiving or delivering the correct voltage signal.

On the Kobelco SK200-8S, this code directly affects the Isuzu AU-6HK1X engine's ability to maintain proper electronic control over fuel injection timing, sensor power distribution, and emission control systems. The main relay serves as the gateway for powering the entire engine management system, making this fault critical for machine operation and preventing unexpected shutdowns during operation.

Common Symptoms

When P0686 is active on your SK200-8S, operators typically experience:

• Check Engine light illuminated on the instrument cluster with potential warning buzzer activation
• Hard starting conditions or extended cranking time, especially during cold starts
• Intermittent power loss to the ECM causing sudden engine stalling during operation
• Complete failure to start despite fully charged batteries and functional starter motor
• Loss of electronic throttle response or erratic engine RPM control

Potential Causes

The most common technical failures triggering P0686 on used SK200-8S excavators include:

• Main relay internal contact failure due to accumulated operating hours (relays typically rated for 100,000+ cycles)
• Corroded or loose connector terminals at the relay socket, especially on machines operated in coastal or high-humidity environments
• Damaged wiring harness between the ECM and relay, particularly at known rub points near the engine mounting brackets
• Faulty ECM power supply circuit including blown fuses in the main power distribution box
• Voltage drop issues from deteriorated battery cables or poor ground connections at the frame rail ground point
• ECM internal driver circuit failure preventing proper relay coil energization

How to Troubleshoot and Fix Code P0686

Step 1: Visual Inspection and Voltage Verification Begin by locating the main relay in the electrical box (typically mounted on the right side of the engine compartment). Using a digital multimeter, verify battery voltage (24V nominal) at the relay power supply terminal with the key switch off. Inspect all connector pins for green corrosion, bent terminals, or heat discoloration indicating previous arcing.

Step 2: Relay Function Testing With the key switch in the ON position, measure voltage at the relay coil terminals. You should see approximately 24V DC when the ECM commands relay activation. If voltage is present but the relay doesn't click audibly, the relay has failed internally and requires replacement (Kobelco part number YN24E00036P1 or equivalent). For used excavators, always check the relay socket contacts for spring tension loss—a common issue after 8,000+ operating hours.

Step 3: Circuit Continuity and ECM Communication Using Kobelco INSITE diagnostic software or compatible scan tool, monitor the ECM's relay control signal in real-time. Check harness continuity from the ECM connector (pins typically C-14 and C-15) to the relay socket using a multimeter set to ohms mode—resistance should be less than 0.5 ohms. Pay special attention to the wiring grommet where the harness passes through the firewall, as this is a known wear point on SK200-8S models.

Step 4: Ground Circuit Verification Measure voltage drop between the ECM ground and battery negative while cranking—readings above 0.2V indicate poor ground connections requiring cleaning and re-torquing to manufacturer specifications (typically 15-20 ft-lbs for main ground lugs).

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Disclaimer: This guide provides general diagnostic procedures for P0686. Always consult the official Kobelco service manual for your specific machine serial number and consider professional diagnosis for complex electrical issues. Improper testing can damage sensitive electronic components.