Q: my macbook pro does not detect external display anymore

This issue seems to be mysteriously connected to Mission control

goto System Preferences -> Mission Control and check "When switching to an application, switch to a space..."

This may fix the issue.

I was having similar issue and was able to solve it by checking the status of GPU on my MBPr used.

Solved by using gfxCardStatus http://gfx.io/ to ensure "discrete only" mode (i.e. use NVIDIA) to detect the external HDMI monitor.

Issue: My MBPr was running with integrated GPU and in this mode, external HDMI monitor will NOT be detected.

Additional Note: Sometimes external monitor will work because you are running applications that will switch on your machine's discrete CPU automatically.

Hope this helps someone.

THIS IS THE SOLUTION!! LOOK NO FURTHER! THANK YOU SO MUCH!

Basically anyone facing the problem of not having their external HDMI monitor/TV detected should just follow the steps by kctang.

Somehow after recent software and firmware updates, the integrated GPU in MBPs that have dual graphics cards (mid-2010 and newer) is not switching to the right one when external HDMI monitors/TVs are connected.

Just download gfxCardStatus and ensure "discrete only" mode (i.e. use NVIDIA) to detect the external HDMI monitor. I'm running OS 10.6.8 and gfxCardStatus v1.8.1. MBP mid-2010 Intel Core i7, 2.66GHz, 8 GB RAM. Connected to 42" Panasonic Viera plasma via no-brand 3rd party mini display port to HDMI adaptor and generic HDMI cable.

This should work 99.999999% if you have similar problems. Thanks again to all who found and passed on the solution!

Guys see my response above (in response to kctang).

The solution 99.999% to work!

sad...I was very excited to finally! a sensible solution and it appears that the fgxcardstatus program (referenced by kctang) is not available for download - not sure if this is temporary and will continue to check back. If anyone has the program or knows another location I can find it please let me know. thanks!

THIS IS THE SOLUTION!! LOOK NO FURTHER! THANK YOU SO MUCH!

 

Basically anyone facing the problem of not having their external HDMI monitor/TV detected should just follow the steps by kctang.

 

Somehow after recent software and firmware updates, the integrated GPU in MBPs that have dual graphics cards (mid-2010 and newer) is not switching to the right one when external HDMI monitors/TVs are connected.

 

Just download gfxCardStatus and ensure "discrete only" mode (i.e. use NVIDIA) to detect the external HDMI monitor. I'm running OS 10.6.8 and gfxCardStatus v1.8.1. MBP mid-2010 Intel Core i7, 2.66GHz, 8 GB RAM. Connected to 42" Panasonic Viera plasma via no-brand 3rd party mini display port to HDMI adaptor and generic HDMI cable.

 

This should work 99.999999% if you have similar problems. Thanks again to all who found and passed on the solution!

 

I tried this on my 13" MacBook Pro (late 2011) and it didn't work.  I don't have any options in gfxCardStatus like that.  The NVDIA options are all greyed out.   I think the 13" MacBook Pros might need a different fix.  Anyone?  Seems like a lot of unanswered questions on here.

The only thing that worked for me was getting a new hdmi cable

It's very strange...I have three vga converters and a number of hdmi/vga cords and an HDMI converter and no combination of them is the fix. However, I have three external monitors and one still works - but the others show no input signal. It's frustrating. Thanks for sharing your experience - I've, searched and searched...

My cable still works though.  I can still use my projector.  I just can't use my TV anymore.

Hi everyone, mini display port to hdmi CABLE is the solution that has worked for myself and many others, please please try it. I recommend the brand I got, NEET. I discovered this back on page 13 of this discussion and its been nearly a year now and ive had no problems connecting to samsung monitors, optoma projectors, and other monitors. Good luck everyone.

I have intermitent connectivity issues with external monitors at home and work (including projectors); usually a reboot gets the job done.  Until this morning...

I wasn't getting any love from any port, thunderbolt or HDMI, and my reboot solution wasn't working.  Searched this forum and nothing made sense or worked.

Then I found this video online http://www.youtube.com/watch?v=41tfRsfNU0A, where the gentleman unplugs the ac adapter from the back of his monitor, waits 10-25 seconds, plugs it back in and his external monitor comes to life (aparently he has to do this everyday).

It didn't work for me at first, but then I tried what internet service providers always suggest: power down my MBP, unplug everything from everything, wait 15-30 seconds, then plug everything back in and power back up. Whaddya know, it actually worked.....

The solution I found was the  Belkin Thunderbolt Express Dock. That connects to my MacBook Pro via Thunderbolt. So now just one cord to my MacBook - the dock connects everything else automatically once plugged in, including my ViewSonic HD monitor. No more issues since getting the Belkin dock. It makes the MacBook think the ViewSonic is an Apple display; and vice versa. I made a short video that shows how it works here: http://www.amazon.com/review/RMYIO8K6HJTUL/ref=cm_cr_rdp_perm

My MBP 13"  has a similar problem, with a Samsung Syncmaster 171s.
However. I have just pluged my phone in to the MbP and hey presto the monitor is working again.
Possibly its a USB power glitch?

I responded to this issue on another similar thread but did not copy it.  I'll attempt to re-create it.

I had a similar problem when I upgraded from Snow Leopard to Mountain Lion two days ago.  I thought I'd have to trash my whole setup (mbp + 2 external HP displays) having unsuccessfully tried what some others have recommended that worked for them.  But then I recalled that when I first attempted to connect my brand new external displays to the mbp three years ago, I had to get a driver for the Diamond USB display adapter, BVU195 I was using & thinking that in the upgrade process, the driver software was deleted, I figured that could be the answer & decided to give it a shot.

The Diamond web site does not have the driver software for this adapter but the download is free from from displaylink.  After downloading the driver, I shut my computer down, disconnected all the hardware & after starting it up again & reconnecting everything, it worked!  So if you're using the Diamond adapter, it's very likely this is what you need to do but don't forget to shut down your computer first as I could not get a signal until I did.

Same problem here, but with an interesting observation. I have a budget HDMI cable that manifests the problem when plugged into the HDMI slot.

1. Wake machine with HDMI cable plugged into HDMI slot... FAIL
2. Plug HDMI cable into Thunderbolt adapter; then plug adapter into Thunderbolt slot... IT WORKS
3. Unplug Thunderbolt adapter; then plug HDMI cable into the HDMI slot... IT WORKS (no reboot required)

Also, I tried the http://gfx.io/ solution without success.

Clearly this is a problem that Apple should address.