Command F2 not working after upgrade to Sierra

I can confirm this is happening in the new Sierra OS. In order to move forward, I wanted to clear up the troubleshooting suggestions (in bold):

If Command-F2 doesn't work

If your iMac doesn't switch to Target Display Mode when you press Command-F2, check these things:

Make sure the iMac you're trying to use as a display supports Target Display Mode.

Of course it supports it, I, and the user reporting the bug, have used it successfully in the past

Make sure you're pressing Command-F2 on the keyboard that's connected to the iMac you want to use as a display.

I can confirm I am pressing Command-F2 on the display I want to use, and getting a blank black screen

In Keyboard System Preferences, if the checkbox is enabled for Use all F1, F2, etc. keys as standard functions keys, the key combination changes to Command-Fn-F2.

I can confirm I am using the F keys as function keys and not standard. To further illustrate that point, when I hit Command-F2, the display attempts to go into TDM, but just returns a black screen.

Make sure your Mini DisplayPort or Thunderbolt cable is connected to both computers.

I can confirm both cables are thoroughly, securely, and unequivocally attached to both machines.

If you're currently logged in on the iMac you want to use as a display, try logging out to return to the login window. Then press Command-F2 again.

I have logged out, logged in, and reset both machines. Problem still exists.

Some older Apple keyboards and third-party keyboards might not allow Command-F2 to toggle display modes. If this happens, use the keyboard that came with your iMac to toggle Target Display Mode on and off.

I am using an Apple keyboard that is less than a year old.

If your iMac is currently started up in Windows, it won't enter Target Display Mode. Target Display Mode isn't supported in Boot Camp.

I am not starting the mac in Windows. I am booting in Mac OS Sierra on both machines.

Now that we have the copy/paste from the troubleshooting guide out of the way. Is there an actual solution to this apparent bug in Sierra?

Hi Luis, thank you for replying.

Mirroring is OFF and yes, I can toggle back in the TDM iMac using Cmd-F2 to a normal screen. As for "what do you see in System Preferences->Displays in the 5K iMac?", you can see that the second iMac shows up, etc., but the actual screen remains blank. If I click on "Gather windows", the second window details appear behind the System Preferences panel, instead of popping up in that second screen. If I pan the pointer to the left, I can see it disappear into the blackness and I know it went all the way to the other side because it takes quite a bit of panning back to get it.

I used to have a second Samsung monitor, some 10 years old, connected to the 5K, that went kaput but worked fine for years. When that one broke, instead of buying a new monitor I remembered I had the other older iMac gathering dust...

Thank you.

Hi prapi01,

Welcome to the Apple Support Communities!

We understand from your post that you have used Target Display Mode before in the past, but after upgrading to macOS Sierra, it is no longer functioning with the Command-F2 command as you would expect.

To troubleshoot this situation, we recommend that you read over the information in the following article, especially the “If Command-F2 doesn’t work” section.

Use your iMac as a display with Target Display Mode - Apple Support

If Command-F2 doesn't work

If your iMac doesn't switch to Target Display Mode when you press Command-F2, check these things:

• Make sure the iMac you're trying to use as a display supports Target Display Mode.
• Make sure you're pressing Command-F2 on the keyboard that's connected to the iMac you want to use as a display.
• In Keyboard System Preferences, if the checkbox is enabled for Use all F1, F2, etc. keys as standard functions keys, the key combination changes to Command-Fn-F2.
• Make sure your Mini DisplayPort or Thunderbolt cable is connected to both computers.
• If you're currently logged in on the iMac you want to use as a display, try logging out to return to the login window. Then press Command-F2 again.
• Some older Apple keyboards and third-party keyboards might not allow Command-F2 to toggle display modes. If this happens, use the keyboard that came with your iMac to toggle Target Display Mode on and off.
• If your iMac is currently started up in Windows, it won't enter Target Display Mode. Target Display Mode isn't supported in Boot Camp.

Cheers!

I have successfully reset the P Ramm of both machines (27" iMac + Mac Mini) and still have the same issue with the black screen.

I connected to my Mini using the "Share Screen" feature in Finder (which has terrible latency and is unworkable, but allows me to actually see what is going on...). Below are screen shots from the Displays tab in About This Mac.

Hello prapi01,

Thanks for the follow-up and letting us know more about the situation and the steps you have tried. At this point, because the previous steps we suggested and the resets you performed have not resolved the situation, we suggest contacting Apple Support directly.

Official Apple Support

Have a great day!

I have the exact same issue. I use my mac mini via my iMac daily and this feature was working correctly immediately before upgrading to Sierra. I also have a black screen when switching over via <cmd> F2. I, too, have reset my SMC and NVRAM/PRAM to no effect. Note that I can access my mac mini through the "Share Screen..." feature in Finder, so I know that it is working. This is a bug.

iMac (27-inch, Late 2012)

Processor 3.4 GHz Intel Core i7

Memory 32 GB 1600 MHz DDR3

Graphics NVIDIA GeForce GTX 680MX 2048 MB

Mac mini (Late 2014)

Processor 2.6 GHz Intel Core i5

Memory 16GB 1600 MHz DDR3

Graphics Intel Iris

VRAM (Dynamic, Max): 1536 MB

Vendor: Intel (-x8086)

Device ID: 0x0a2e

Revision ID: 0x0009

Metal: Supported

macOS 10.12.1 on both systems.

No update yet, but I am hoping for an fix in the next Sierra update (I am currently on 10.12.1). In the meantime, I encourage you to file a bug report to help Apple diagnose the issue and to confirm that multiple users are having issues (bug report link above) Provide your system details, e.g.

Hardware Overview:

Model Name: iMac

Model Identifier: iMac13,2

Processor Name: Intel Core i7

Processor Speed: 3.4 GHz

Number of Processors: 1

Total Number of Cores: 4

L2 Cache (per Core): 256 KB

L3 Cache: 8 MB

Memory: 32 GB

Boot ROM Version: IM131.010A.B09

SMC Version (system): 2.11f14

NVIDIA GeForce GTX 680MX:

Chipset Model: NVIDIA GeForce GTX 680MX

Type: GPU

Bus: PCIe

PCIe Lane Width: x16

VRAM (Total): 2048 MB

Vendor: NVIDIA (0x10de)

Device ID: 0x11a3

Revision ID: 0x00a2

ROM Revision: 3707

Metal: Supported

Displays:

iMac:

Display Type: LCD

Resolution: 2560 x 1440

Pixel Depth: 32-Bit Color (ARGB8888)

Main Display: Yes

Mirror: Off

Online: Yes

Automatically Adjust Brightness: Yes

Built-In: Yes

Found the solution. Still a bug and will need to be updated soon so please report to apple.

Restart both machines and on the chime, hold Cmd+ALT+P+R on the restart of each machine. This resets your P Ramm.

Only now coming across this thread, I would like to add my $0.02:

1) It is clear that the iMac is responding to Command-F2, so the issue seems to be the Mac Mini's outputting some display resolution that is wrong for the iMac.

2) This is particularly difficult because of the headless nature of the Mini. If a user can do screen sharing to the affected mini, it would be interesting to see what one gets from System Preferences->Displays in this case - in particular, to confirm that the Mini detects the display is attached, and which resolution it is outputting to it.

FWIW, I use target display mode just about everyday, albeit in a different situation - to add a second display to my rMBP - and it works flawlessly. Again, the headless nature of the Mini seems to be the crux of the matter.

All the others affected so far appear to be using *headless* minis.

Your experience seems unique in that you are unable to use target display mode from an imac to another.

You should be able to try the proposed solution (reset NVRAM on both computers, but more specifically the one that is the source, as the other one appears to correctly enter TDM).

Structurer wrote:

I have successfully reset the P Ramm of both machines (27" iMac + Mac Mini) and still have the same issue with the black screen.

I connected to my Mini using the "Share Screen" feature in Finder (which has terrible latency and is unworkable, but allows me to actually see what is going on...). Below are screen shots from the Displays tab in About This Mac.

This is a step forward, I hope.

(The fact that it says "Built-in display" in the mini is awkward, but let's proceed :-))

Are you doing this from the same imac that is supposed to work in TDM? If you could temporarily have access to a third mac or a directly attached display this might help in sorting this out.

The problem if this is the same imac is that while you are doing this share screen session the imac as a display is not available.

Anyway, let's try. It appears that the mini is outputting the wrong resolution.

If you click on "Default for display" and then OPTION-click on "Scaled", do you see other resolutions? Could you select 1920x1080?

I have a brand new 13" MBP and Late 2015 5K iMac - I bought the **** USB-c adapter and **** cable from the apple store. I can't get it to work. I went across the street to my neighbor with same setup (brought MacBook Pro, adapter and cable and was able to use his with no troubles. I think I have it to my keyboard or iMac... I have reset PRAM, left it unplugged for 10 min. All without success. I have connected my iMac to external HP monitor and it worked fine so I think the DisplayPort seems to be working. I don't know what else to do....

It is not clear what you are trying to do.

The original poster had a problem with Target Display Mode - i.e. using the iMac as a display for another mac.

Your 5K iMac does NOT support Target Display Mode, so in your case if that is what you are trying to do, there is no chance. If something else, please start a new thread and describe clearly what you want to do and how it is failing for you.