Yosemite does not recognize external displays...

Intermittent problems with video or video and sound.

From my iMac, I go through a planex mini display port HDMI, to my AVR, to my TV. I think, if my computer goes to sleep before I use the second monitor, then the iMac will not recognize there is a 2nd monitor after it wakes up. Sometimes sound works with no picture. If i restart, then my 2nd monitor works. The AVR is getting a signal, cause it shows a black screen instead of the DENON screen it would be showing if there is no signal. But the iMac shows no 2nd monitor under system settings.
Is there anyway to not restart the iMac to get it to see the 2nd monitor when this happens? What a pain.

Dear All,

Recently I bougth MacBook Pro 15" Mid 2015. At home I have Monitor Dell U2715H.

When I connected Dell's cable to Mac Thunderbolt port. the monitor was recognized, but entered into energy save mode and message was - not signal.

The solution that I've found is to disable DP1.2 from monitor's menu. This option reverts to DP1.1 and it works now.

Cheers

I have same problem, i tried many steps but nothing happened 😟 until I found a solution for it by mistake😁! first let me tell you about my setup and problem.

I'm connecting my 32" tv display by hdmi wire to my imac using not original thunderbolt to hdmi adapter! and everything work fine! for the first time after a while my mouse get glitch and my imac can't recognize external displays!

solution was too simple! just unplug hdmi wire from my tv then return it.

now that problem comes back again and that solution didn't work anytime!

but when I plug and unplug my hdmi wire and the adapter ( the light was off in my room so it was dark ), I notice there is a spark in the hdmi adapter when it hit the imac aluminium back by mistake, then i realized there was a "Static electricity" in the hdmi wire & the adapter! So I disconnected ( the wire ) from the tv and touch it to discharge electricity then reconnecting again, everything work fine😊.

I think the static electricity confuse my imac device!!!!

hope this help someone, and be careful about this nonsense solution.😕

I have same issue and tried all possible solutions in this forum with no luck. All my cables work well on my Mac Mini but not on my MBA.

My problem solved by using USB 3 to HDMI cable from Cablematters, bought it in Amazon. You need to install the Displaylink driver to make it work. I connect my MBA 11" (mid 2012) with two external displays (LG 24MP77), and has latest Yosemite (10.10.5) installed. This solution also works well on my docking LandingZone Pro.

Similar issues in 10.10.5 update. My cable was purchased at an apple store, was working fine (connected my Macbook air to a Samsung monitor) until the update. Went to the store and they tried several other cables and confirmed it was a software update issue.

Having read through many of the posts it seems that this is a recurring problem with software updates at least in Yosemite. This is my first post on these forums, so perhaps I'm being a little backward, but why isn't this problem being fixed? Why do we have to resort to so many workarounds? Come on Apple, this isn't good enough! Never had any problems with connecting PCs to monitors!

I understand that El Capitan is being released at the end of this month. Will it fix these problems?

Yay!🙂

Worked forst time on a Philips 234E - even with a VGA cable through a poxy adapter

I unplugged the mini HMDA and then put it back in again

Thankyou

I crowed too soon - the monitor is no longer recognised again as soon as the Mac goes on standby. After that, the only way to get it recognised again is to ... remove the monitor lead from the back of the box and then plug it in again. And again. And again. 😢 Not a long term solution.

This is really poor - if even a linux distro can recognise this monitor first time and get the resolution and screen proportions right, what's going on at Apple? Get your act together please.

I encountered a similar problem with my brand new MacBook Air 13" (OS 10.10.5) yesterday: After connecting it to an Epson projector at my school (using an Apple Thunderbolt to VGA adaptor cable), a fully blank screen (without any desktop symbols or the dock) was displayed on the projection screen. At the same time, the MacBook's monitor displayed all the standard things. However, strangely enough, after starting a PowerPoint presentation, this was displayed flawlessly on the wall screen!

I have been teaching for more than 20 years, and during this time I have used a number of iBooks and MacBooks of various generations. Not once did I encounter a similar problem. I am slightly frustrated with this unusual Apple experience and will have to experiment with some of the advice given here and on other forums.

After scouring several support forums I came across SwitchResX which works a treat. The problem is slightly wierd in that the startup apple is not distorted (so obviously the computer does recognise the settings) and then the login screen is distorted. SwitchResX allows all that to be overriden and system monitor resolution settings can be specified by the user.

Just posting this in case it helps - solved my problem by using a mini display port to vga adapter.

I won't be rushing in to apply the next OS update that's for sure.

I use a mini-display to VGA adapter pretty much everywhere. I use it on projectors and on external monitors (at least 5 or 6 different ones, each with a different adapter).

This has suddenly stopped working on my MacBook.

It's intermittent, though it's rather rare to have it work. Sometimes I leave the adapter connected for the whole day while I work and it will randomly work. If I disconnect the port and attempt to reconnect it, it will fail to detect the monitor again.

I thought this was a hardware issue, ran the Apple Hardware Test and no problems where found. I've cleared NVRAM and restarted SMC to no avail. I've even started on Safe mode and deleted the /Library/Preferences/com.apple.windowserver.plist file. It's still the same. There was a thunderbolt update today which I installed and still no dice.

However, an Apple thunderbolt display at the office works flawlessly. What gives?

I use a mini-display to VGA adapter pretty much everywhere. I use it on projectors and on external monitors (at least 5 or 6 different ones, each with a different adapter).

This has suddenly stopped working on my MacBook.

It's intermittent, though it's rather rare to have it work. Sometimes I leave the adapter connected for the whole day while I work and it will randomly work. If I disconnect the port and attempt to reconnect it, it will fail to detect the monitor again.

I thought this was a hardware issue, ran the Apple Hardware Test and no problems where found. I've cleared NVRAM and restarted SMC to no avail. I've even started on Safe mode and deleted the /Library/Preferences/com.apple.windowserver.plist file. It's still the same. There was a thunderbolt update today which I installed and still no dice.

However, an Apple thunderbolt display at the office works flawlessly. What gives?

My first post...ever. I have a MBP mid 2012. Using two LG monitors, one with an ersatz thunderbolt to DVI adaptor and the other with a J5 Create USB 2.0 to DVI adaptor. Used both monitors without any problems until the Yosemite upgrade. The second monitor (the one with the J5 adaptor) went blank and it wasn't recognized in System Preferences. I read a bunch of posts and saw that someone had success updating the J5 Create driver. That also worked for me.

My MacBook Air is now working together perfectly with the Epson projectors at my school. Unfortunately, I am not able to reconstruct what piece of advice from the various forums finally did the trick. But it was certainly nothing "sophisticated" like resetting PRAM or SMC or replacing cables or similar stuff. I am still using the original Apple Thunderbolt to VGA adaptor cable, and I believe the only thing I really had to do to solve the problem was use the Option key within the monitor system preferences as described here:

http://osxdaily.com/2014/03/31/detect-displays-mac-os-x/

I sincerely hope this helps others too.

Just in case this helps anyone:

I use an external Dell monitor with my MacBook Pro (Retina) connected via Mini-Displayport-to-DisplayPort cable. The display stopped working after an OS update; my screen would flash and my computer would freeze up, and the computer would intermittently detect the monitor for a split second and then lose it over and over again.

Drove myself crazy trying all of the above solutions except swapping out the cable because I couldn't believe that it would suddenly be a cable problem when this cable had been working perfectly for over a year. I even upgraded to El Capitan even though I didn't want to yet in the hopes that it might solve the issue (it did not). Finally, I tried purchasing a new cable. It didn't work.

Well, it turns out that DisplayPort cables aren't particularly standardized. As of today, VESA lists one manufacturer of Mini-DisplayPort-to-DisplayPort cables as certified compliant (Accell). I purchased an Accell cable. Everything worked perfectly after that. Luckily, they're not that expensive as cables go.

http://www.displayport.org/cables/how-to-choose-a-displayport-cable-and-not-get- a-bad-one/

(includes a link to VESA site page that lists all certified compliant DisplayPort cables)

Perhaps what happened is that Apple changed something so that many cables which used to work just fine before no longer work, and a strictly VESA compliant cable is now necessary.

So even though it seems ridiculous, replacing the cable could work... if you get the right cable.