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Mac not detecting your external monitor? How to fix a dark, mirrored or blurry screen

By the AI Repair team · Updated

You plug in a monitor and nothing happens: the screen stays black, says No signal, or simply shows a copy of your Mac’s screen. Most of the time the cause is the monitor’s input, the cable or adapter, a hub in between, or a limit on how many monitors your Mac can use. These steps go from the quickest check to the rarer causes, and end with a blurry or oddly sized picture.

In short

Adding a second or third monitor? First check how many your Mac can use: some MacBooks take only one, and no cable changes that. Otherwise, check that the monitor is on and set to the input you plugged into, push the cable in firmly at both ends, and plug the monitor straight into your Mac instead of through a hub. Then open System Settings › Displays (System Preferences › Displays on macOS 12), hold Option and click Detect Displays. Still dark? Put your Mac to sleep and wake it, or restart it.

Adding a second or third monitor? Check first how many monitors your Mac can use (step 7): a MacBook Air with the M1 or M2 chip, for example, takes only one, and no cable changes that.

1. Check the monitor and its cable

  1. Make sure the monitor is plugged in and switched on. Its power light should be on.
  2. With the monitor’s buttons, or the TV’s remote (Input or Source), choose the input you plugged into: HDMI, USB-C or DisplayPort. If it has an automatic input setting, turn it off.
  3. Unplug the video cable at both ends, wait 10 seconds, then push it back in firmly.
  4. If you have another cable, try it. Some USB-C cables only charge and carry no picture.

Apple adds that the cable or adapter must support the monitor’s resolution and refresh rate, and that a USB-C adapter needs to support DisplayPort or Thunderbolt video (Apple’s guide).

2. Plug it straight into your Mac

If the monitor goes through an adapter, a hub or a dock, plug it straight into your Mac for now, or try another port on your Mac. Before you unplug a hub, eject any drive on it: in a Finder window, click the eject button next to the drive in the sidebar. With a MacBook’s lid closed, it needs to be plugged into power, with a keyboard and mouse connected.

3. Allow the monitor to connect

With macOS 13 or later, a Mac laptop with Apple silicon may ask whether to allow a new accessory, such as a hub or monitor, to connect (macOS 12 doesn’t ask). Look at the top right of the screen for the message, and click Allow. If you clicked Don’t Allow before, eject any drive on the hub, then unplug the monitor or its hub, wait a few seconds and plug it back in: your Mac asks again (Apple’s guide).

The setting that decides when your Mac asks is in System Settings › Privacy & Security: scroll down to Allow accessories to connect. On newer macOS, click Accessories first.

4. Ask your Mac to look for the monitor

  1. Choose Apple menu › System Settings, then click Displays in the sidebar. You may need to scroll. On macOS 12, choose Apple menu › System Preferences, then click Displays.
  2. Hold down the Option key until a Detect Displays button appears where Night Shift was.
  3. Click Detect Displays, then let go of the Option key, and wait a few seconds.

5. Put your Mac to sleep and wake it

Save anything you’re working on, then choose Apple menu › Sleep. Wait about 10 seconds, then press a key or click the mouse to wake your Mac. On a MacBook you can instead close the lid for a few seconds, then open it again.

6. Restart your Mac

Save your work and close your apps, then choose Apple menu › Restart. Leave the monitor plugged in and switched on while your Mac starts up.

7. Check how many monitors your Mac can use

A monitor stays dark when your Mac already has as many as it can use, and no cable changes that. For example:

  • A MacBook Air or a 13-inch MacBook Pro with the M1 or M2 chip takes one external monitor.
  • A MacBook Air with the M3 chip takes two, but only with its lid closed (with macOS Sonoma 14.3 or later; a 14-inch MacBook Pro with the M3 chip needs 14.6): connect a keyboard, a mouse and power, plug in the first monitor, close the lid, then plug in the second. If you open the lid again, the second monitor goes dark (Apple’s steps).
  • A Mac mini with the M1 chip takes two, but only one through its USB-C ports: plug the second into the HDMI port.

For your Mac, choose Apple menu › About This Mac to see its model and year, then find its Tech Specs on Apple’s specifications page: the display support section says how many monitors it can use.

A dock with DisplayLink can add more monitors than your Mac takes by itself. It needs Synaptics’ free DisplayLink Manager app, from Synaptics’ website, and your permission for that app to record the screen, in Privacy & Security › Screen & System Audio Recording (Screen Recording on macOS 13 and 14; Security & Privacy › Privacy › Screen Recording on macOS 12).

If the monitor shows the same picture as your Mac

Your Mac is mirroring: both screens show the same thing. To use the monitor as extra space instead (Apple’s guide):

  1. Choose Apple menu › System Settings, then click Displays in the sidebar.
  2. Click the name of your monitor.
  3. Click the Use as menu and choose the Extended option, not Mirror.

On macOS 12 Monterey: in System Preferences › Displays, click Display Settings, click your monitor in the list on the left, then choose Extend Display from the Use as menu.

If the picture is blurry, too big or too small

  • Blurry: in System Settings › Displays, click the monitor’s name and choose the size marked Default. Apple notes that a monitor’s own resolution is often sharper than a scaled one. On macOS 12, click Display Settings, choose the monitor and then Default for display.
  • Everything too small or too big: choose a size towards Larger Text or towards More Space. On macOS 12, next to Resolution choose Scaled, then a size.
  • Jerky or flickering: next to Refresh rate, choose the highest number. A 4K monitor stuck at 30 Hertz usually means the cable or adapter can’t carry more: an HDMI cable marked Premium High Speed, or a USB-C to DisplayPort cable, can.
  • Sideways: next to Rotation, choose Standard.

If the screen goes black after you change a setting, wait 15 seconds: your Mac may go back to the old setting by itself. If it doesn’t, press Esc (Apple’s guide).

When it may be the hardware

Lines, cracks or spots in the picture, or your Mac’s own screen staying dark at full brightness, are a fault no setting fixes: have the screen checked by Apple or a repair shop. A monitor from another maker that stays dark, and whose brightness you can’t change with its own buttons, may have no power or a fault: look for a firmware update on its maker’s website, or ask the maker’s support (Apple’s guide).

Advice you can skip

  • Resetting the SMC or NVRAM on a Mac with Apple silicon. Apple says those steps aren’t needed there.
  • Forcing a resolution with another app or a Terminal command. When a size you choose in Displays settings leaves the screen black, your Mac can switch back by itself; a setting forced another way may leave you looking at a black screen.
  • Buying a new monitor before you’ve checked the cable, the input and how many monitors your Mac can use.
  • Buying a “multi-monitor” hub for a Mac that takes one monitor. A plain hub can’t add a second picture; only a DisplayLink dock or adapter, with its app, can.

Common questions

Why does my monitor say No signal when it’s connected to my Mac?

The monitor isn’t receiving a picture on the input it’s set to. Choose the right input with its buttons, push the cable in firmly, and plug it straight into your Mac. A cable that only charges, an adapter that can’t carry video, or a Mac already at its monitor limit also give No signal.

How many external monitors can my Mac use?

It depends on the model. A MacBook Air with the M1 or M2 chip takes one, for example, and more powerful Macs take two or more. Apple lists the number in each model’s Tech Specs, under display support.

Why does my second monitor show the same thing as my laptop?

Your Mac is mirroring its screen. In Displays settings, click the monitor’s name, open the Use as menu and choose the Extended option, so the monitor becomes extra space.

Why is my external monitor blurry on my Mac?

It’s usually set to a size that isn’t its own. In Displays settings, click the monitor and choose the size marked Default. If text is then too small, use your apps’ zoom, with ⌘ and plus, rather than a blurry size.

Can I use my iPad as a second screen for my Mac?

Yes, with Sidecar. Keep the iPad nearby with Wi-Fi and Bluetooth on, signed in with the same Apple Account (Apple ID) as your Mac. On your Mac, click Control Center, then Screen Mirroring, and choose the iPad.