Fix a Flickering Screen Issue with External Displays on Mac After Updating Mac OS X
Having recently went through the process of updating a handful of Macs to the latest versions of Mac OS X as part of a gifting tech support campaign, several Macs developed an odd flickering display problem that was only visible when an external monitor was connected to the Mac. On an 11″ MacBook Air, the screen flickers with any DVI or VGA display hooked up with the standard Min-iDisplayPort adapter, and displays only a white-black noise screen, like an ancient TV without an antenna. Meanwhile on a newer MacBook Pro, the problem exhibited as external display flickering with anything from DVI connections to HDMI, and with different brand monitors, but the screen would display properly just with a persistent randomized flicker. The video below demonstrates the MacBook Air flickering on and off with a noisy picture:
(Excuse the vertical video formatting, someone didn’t follow this important advice for recording with the iPhone camera!)
Because each of the Macs behaved perfectly normal with external displays prior to the Mac OS X update, and the problem spans between two totally different Macs and with different connection types and even monitor brands, this suggests something went awry during the upgrade process. That’s good news, because that also means the problem was likely not to be simultaneous concurrent hardware failure – a truly rare event that would be even moreso with two unrelated machines failing side by side. Fortunately, a simple SMC reset resolved the problem completely on both Macs, and external displays again worked as they should post-SMC reset.
Resolve Monitor Flickers by Resetting the System Management Controller of a Mac
This is how you reset the SMC on a portable Mac that has a built-in battery (not a removable battery), like a MacBook Air, or new MacBook Pro with Retina display. You can find details for SMC resets on other Macs here, and Apple has directions for other machines here too.
- Disconnect the external display from the Mac
- Shut down the Mac so that it is turned OFF (not sleeping)
- Connect the MagSafe power adapter to the MacBook
- Using the built-in keyboard on the MacBook, press and hold down the Shift+Control+Option+Power buttons at the same time (see the image below for details) and hold them for about 2 seconds, then release all keys together
These are the keys to press and hold simultaneously on the MacBook Pro / Air keyboard to reset the SMC:
When that is finished, boot the Mac and reconnect the display:
- Now press the Power button on the keyboard to boot the Mac as usual
- Wait until the Mac is booted and reconnect the external display
Everything should be working as usual, no more monitor screen flickering, no more noisy screen displays, just the Mac with a normal looking external display.
Beyond display issues, resetting the System Management Controller can also resolve many other problems related to power management and hardware, both that have occurred post system updates or seemingly at random. We’ve covered several other issues like this in the past, including a suddenly missing battery and fans blaring on portable Macs, inability to put a Mac into sleep mode, and fan noise and heat issues that have popped up after installing a system update. Because there is little harm in resetting the SMC beyond losing some simple Energy settings that you can reconfigure within System Preferences, it’s worth trying yourself before taking further steps to resolve hardware problems with any Mac, and it’s actually a common step recommended by Apple Genius staff through phone support, and even at the Genius Bar in an Apple Store.
If you or anyone you know happens to run into the screen flicker issue after an Mac OS X update applied to a Mac, try this SMC fix first, it only takes a moment and resolved the problem quickly in these cases, and just may work for you too.