Mac Is Not Detecting an External Display and How to Fix It

Try the following fixes, in order, to try to get your Mac to recognize the external display.

1. Double-check your cables. The first thing you should do is make sure your cables are both correct and secure. Confirm that any cords running between your Mac and the display are firmly connected to both, and ensure that the display is plugged in.
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3. Make sure you've got enough power. If you're using a MacBook, try plugging it into power. Some power-saving settings restrict certain processes to extend battery life, and if your display isn't getting enough power, it won't work.
4. Disconnect and then reconnect your cables. If you've already checked your connections and know that your monitor is getting enough power, try unplugging all your cables (including the power cable), wait about 30 seconds, and then plug them back in. Restarting the external display may give your Mac a fresh shot at recognizing it.
5. Make sure your Mac supports more than one additional monitor. You can check the supported number of displays by clicking the Apple icon System Settings > Help > your MacBook > and looking under Display Support or Video Support.
6. On older macOS versions, click the Apple icon > About This Mac > Support > Specifications > Graphics and Video Support.
7. Ensure that any adapter is compatible. If you're using an adapter, make sure that your monitor and cable combo is compatible with your Mac. Some Macs use only USB-C or both Thunderbolt and USB-C connections, and some Thunderbolt variations require a Thunderbolt-specific adapter.
8. In the case of a dock, check its manufacturer's support site and download new drivers, if applicable.
9. Check your resolution. Check resolution settings by going to System Settings > Displays. Press and hold down the Option key. This will reveal a hidden Detect Displays option at the bottom right. Click this button so that your Mac can scan for a connected display.

Choose different resolutions. Still in Resolution settings, Ctrl+click a resolution on the list > and choose Show Thumbnails. Select a thumbnail other than the default option to see if this prompts a change.

1. Deselect automatic brightness adjustments. Under Brightness, uncheck the box next to or deselect Automatically adjust brightness and manually brighten using the toggle. If the external display's brightness is too low to register, this could resolve that issue.
2. Give It a rest for a few seconds. Put your Mac to sleep for a moment, and then wake it back up to see if anything changes in the Display preferences.
3. Restart your Mac. Go ahead and restart your Mac, but keep the external display connected. Sometimes this will clear whatever is preventing your Mac from detecting the monitor so when it powers back on, it can complete the connection process.
4. When all else fails, start all the way over. Disconnect all cables, power off your Mac and the external monitor for a minute or two, and then reconnect the cables and turn everything back on. Repeat the steps for the Display settings if the monitor still doesn't work.