Hitachi ZH200-5A Fault Code P0685 – Meaning and Fix

What is Hitachi ZH200-5A Fault Code P0685?

Fault Code P0685 indicates an "ECM/PCM Power Relay Control Circuit Open" condition, meaning the Engine Control Module (ECM) has detected a failure in the power relay circuit that controls the main electrical supply to the engine control system.

This code signifies that the ECM cannot properly control the power relay responsible for maintaining electrical power to critical engine management components. On the Hitachi ZH200-5A excavator, this relay controls the ignition circuit and provides switched power to fuel injectors, sensors, and the ECU itself. When this circuit fails or opens unexpectedly, the engine may experience immediate shutdown, fail to start, or lose vital sensor data mid-operation. This is critical because the ZH200-5A relies on continuous ECM power to manage fuel delivery, emissions controls, and hydraulic synchronization with engine load demands.

Common Symptoms

When fault code P0685 is active on your Hitachi ZH200-5A, you may experience:

• No-start condition – engine cranks but won't fire or immediately shuts down after starting
• Check Engine Light or malfunction indicator lamp (MIL) illuminated on the instrument cluster
• Intermittent engine stalling during operation, particularly after warm-up or under load
• Loss of throttle response or sudden power loss during digging/swinging operations
• Complete electrical shutdown of engine control systems while hydraulics may remain partially functional

Potential Causes

The most common technical causes for P0685 on used Hitachi excavators include:

• Failed ECM power relay (located in the main relay box near the battery compartment)
• Corroded or loose relay socket connections due to moisture intrusion or age-related oxidation
• Broken or damaged wiring harness between the ECM and power relay, often at chafe points near the engine mount or chassis frame
• Blown fuse in the ECM power supply circuit (typically 15-20A rated)
• Faulty ECM internal relay driver circuit causing inability to energize the external relay coil
• Poor ground connections at ECM mounting points or battery negative cable terminals
• Previous repair attempts that created intermittent connections or improper relay replacement

How to Troubleshoot and Fix Code P0685

Step 1: Visual Inspection and Relay Testing

Locate the ECM power relay in the main electrical panel (consult service manual for exact location). Remove the relay and inspect the socket for corrosion, bent pins, or moisture. Test the relay using a multimeter: apply 12V to the coil terminals (typically pins 85 and 86) and verify continuity between load terminals (30 and 87). Replace if no click is heard or continuity fails.

Step 2: Check Fuses and Power Supply

Inspect all fuses in the ECM power circuit using a multimeter set to continuity mode. Verify battery voltage (12-14V) reaches the relay input terminal with ignition on. On used excavators, check for voltage drop across corroded fuse holders—even 0.5V drop can cause issues.

Step 3: Wiring Harness and Ground Verification

Trace the wiring harness from ECM to relay, checking for abrasion, cuts, or pinch points common near the turntable rotation bearing. Use a multimeter to verify continuity and check for short-to-ground conditions. Measure ground resistance at ECM mounting bolts—should be less than 0.5 ohms. Clean all ground connections with wire brush and apply dielectric grease.

Step 4: ECM Control Circuit Testing

With relay removed, use a multimeter to measure voltage at relay socket coil terminals while key is turned on. Should read battery voltage momentarily. If no voltage present, suspect ECM internal driver failure or damaged control wire. This requires Hitachi diagnostic software (Dr.EX) for deeper ECM testing.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Hitachi service manual for your specific machine serial number. If you're uncomfortable performing electrical diagnostics, contact a certified Hitachi technician. Improper repairs can cause expensive ECM damage or create safety hazards.