External monitor not detected on a Mac? 7 fixes to try

2 min read · 7 steps · Updated 2 October 2026

Short answer

Check that the power and video cables sit firmly at both ends. Then open System Settings, Displays, hold the Option key and click Detect Displays. If the screen stays dark, restart the Mac and try another cable or adapter.

A second screen that stays black or shows No Signal is usually a cable, adapter or input problem. The Mac also has a hidden button that looks for displays again.

Menu names are from recent versions of macOS, where the settings app is called System Settings. The pictures are our own screenshots from macOS Tahoe 26 on a MacBook Pro.

Step by step

  1. Check the cables at both ends

    Make sure the power cable and the video cable of the display are connected securely and correctly at both ends. Check that the display is switched on and set to the right input, such as HDMI 1 or USB-C.

  2. Part of the Mac System Settings window: Use Detect Displays

    Use Detect Displays

    Open System Settings and click Displays. Press and hold the Option key. A button called Detect Displays appears. Click it.

  3. Restart the Mac

    Restart with the display connected. If the Mac does not react, hold the power button for about 10 seconds to switch it off, then start it again.

  4. Check the cable and the adapter

    With Thunderbolt and USB-C, the cable or adapter must support the resolution and refresh rate of the display. A cable that only charges cannot carry a picture. Try another cable, and plug the display straight into the Mac without a dock.

  5. Turn up the brightness of the display

    If the display is detected but dark, its brightness may be turned down. Use the buttons on the display or the brightness slider in System Settings, Displays.

  6. Change the resolution

    If the picture is blurred or the wrong size, select the display in System Settings, Displays and choose another resolution. The native resolution of the display is usually the sharpest.

  7. Update macOS

    macOS updates also contain firmware updates for Apple displays.

Good to know

Each Mac model supports a limited number of external displays. If a second or third display stays dark, check the technical specifications of your model on Apple's website.

To test the display itself, connect it to another computer.

Setting the resolution: how to change the screen resolution on a Mac.

Sending the picture to a television without a cable: how to mirror a Mac screen to a TV with AirPlay.

In short: Cables, input, Option and Detect Displays, restart, other cable, brightness, resolution, update.

Sources
  • Apple Support: If your external display is dark or low resolution

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