Pattern to record: A USB-C accessory or display fails while the desktop’s power state is being investigated.

Use the correct Mac power design

Identify the iMac before tracing power: the 24-inch Apple-silicon iMac uses its dedicated external adapter, while many earlier models take mains power into the chassis. Record whether the display, fans or other functions show any startup activity. iMac USB-C ports serve supported peripherals and displays; they are not the desktop’s normal input-power connection.

Power and data precautions

The external adapter or internal mains supply can contain hazardous voltage; display removal requires trained handling. Do not infer a failed charging controller merely because a USB-C accessory is undetected.

Model-specific checks

1. Identify the external-adapter or internal-supply design, then distinguish startup activity from a built-in display or backlight fault.
2. Disconnect nonessential peripherals, use the specified mains or adapter connection and check whether the Mac starts independently of the accessory.

Repair and startup verification

If the checks support this cause, diagnose a USB-C peripheral path separately if the Mac starts; investigate the real power supply and startup sequence if it does not. Verification should cover cold start, built-in display, brightness, processor load, sleep and restart.

Reference: Apple support guidance; Apple support guidance.

For an assessment of your iMac, contact AEPC in Burswood or call 027 908 8880. Provide the model and the conditions that reproduce this problem. Workshop visits are by appointment.