Your Mac isn't detecting your external monitor

Why it happens

Most of the time the Mac isn't getting a signal at all: a loose cable, the wrong input on the monitor, or an adapter that doesn't carry video. Sometimes the signal is fine but macOS simply hasn't noticed the screen yet. Detect Displays makes it look again.

How to fix it

1. Push both ends of the cable in firmly and make sure the monitor is switched on. A small power light on the monitor should be lit.

2. Use the buttons on the monitor to pick the input you plugged into, such as HDMI 1, HDMI 2, USB-C or DisplayPort. Many monitors don't switch by themselves.

3. Open System Settings and click Displays. Hold down the Option key: the Night Shift… button at the bottom changes to Detect Displays. Click it while you keep holding Option.

   Displays›Detect Displays

4. Still nothing? Leave the monitor plugged in and choose Restart from the Apple menu. The Mac checks for screens again as it starts up.

Still not fixed?

1. Try another cable, and plug it straight into the Mac instead of through a hub or adapter. Some USB-C cables only charge and copy files and can't carry a picture.

2. Check how many screens your Mac supports. Some models, including many MacBook Air models, can drive only one external display. About This Mac shows your model, and Apple's tech specs page for it lists the limit.

   Apple menu›About This Mac

3. If the monitor stopped working after a macOS update, check Software Update for a newer fix and install it.

   General›Software Update

4. Test the monitor with another computer, and another monitor with your Mac. If the monitor fails everywhere, it's the monitor. If every screen fails on one port of your Mac, the port may be faulty, which is hardware: contact Apple Support or a repair shop.