Fault code P0685 – a step-by-step guide to diagnosing and fixing it

Error code P0685 is displayed and the check engine light is on - what does this mean?

The DTC P0685 indicates that the OBD system has detected a problem with the ECM/PCM power supply relay control circuit. This fault code occurs when the engine control unit detects irregularities in the power supply or relay control. For various manufacturers, this code may also be referred to as "ECM/PCM Power Relay Control Circuit/Open" or "Engine Control Module Power Relay Control Circuit".

What can cause this error code?

• Defective ECM/PCM power supply relay
• Damaged or corroded plug connections on the relay
• Blown fuse in the ECM/PCM power supply
• Broken or damaged cable connections to the motor control unit
• Defective motor control unit (ECM/PCM)
• Loose or corroded earth connections
• Overloading of the electrical system
• Defective relay socket in the fuse box

What symptoms occur in parallel with fault code P0685?

• Starting problems: The vehicle is difficult or impossible to start
• Engine misfires: Irregular engine running or sudden stalling
• Loss of power: Reduced engine power while driving
• Electrical problems: Failure of various electrical systems
• Unstable idling: Engine runs erratically at idle speed
• Check Engine Light: Continuously illuminated check engine light

How can the problem of error code P0685 be rectified?

• The repair requires a systematic diagnosis of the power supply circuit of the engine control unit.
• Checking and replacing the ECM/PCM power supply relay in the fuse box, including testing with a functional replacement relay
• Check and clean all plug connections on the relay and motor control unit, followed by application of contact spray in case of corrosion
• Measuring the power supply with a multimeter and replacing blown fuses in the ECM/PCM power supply
• Checking the cable connections between the relay and motor control unit for continuity and repairing damaged cables
• Checking and cleaning all ground connections of the engine control unit and ensuring tight connections
• Diagnosis of the engine control unit itself using professional diagnostic equipment and, if necessary, replacement if defective