Okay, so I've spent almost the entire last day on this problem, and I've googled the crap out of it, I hope someone here can help me.
First off: Yes, I've cleared the NVRAM/PRAM several times and I've reset the SMC more than once too.
I have a setup with two displays on my desk that are connected to my desktop PC. I sometimes used one of those displays with my MBP via HDMI. A couple of days ago, I've decided to use both of those with my MBP and an external keyboard/mouse just to have a more tidy experience on my desk.
So I got two DisplayPort to HDMI cables to realise this extravagant folly.
Now, when I plug either of them in, nothing happens.
The (internal) display will flicker for a second, but the external displays are not detected. Neither via the old "hold option and click detect displays in Preferences->Displays" nor via "put Mac to sleep and wake it up". I've cleared the NVRAM and reset the SMC several times. I've booted up in safe mode and tried it. No dice. They don't show up in the system information box either.
To be clear, I'm not trying to connect both displays at this time. Neither of them works with neither cable.
The cables are not faulty, as I've tried both cables and both monitors with my MacBook Air Mid 2011, and they work just fine in all possible combinations.
Both displays work just fine with the MBP directly via an HDMI cable or with my mini DisplayPort->VGA adapter.
So, surely, it must be some software issue.
Now, I've tried to use one of those hellish cables to connect my MBP to my TV, and –voila– it worked. So my MBP is indeed capable of using those cables.
So, I've ruled out
a) the cables
b) the displays
and c) the notebook as a source for my woes.
Neither is it the combination of a & b (works with MB Air), a & c (works with TV), or b & c (works via HDMI and mDP->VGA).
What am I missing?
Here's the displays, sorry for not including them in the original post:
- BenQ XL2411
- Samsung SyncMaster P2770
The TV that for some reason works with the cables and the MacBook is a Panasonic TX-32ASW504. I'll try and go around town and plug my Macbook into other people's HDMI to test out other displays.
I'm running macOS Sierra 10.12.2 on a MacBook Pro Retina 13'' Mid 2014.
Best Answer
So it looks like you have systematically ruled out all hardware issues on all components, so you're right and it sounds software related. And since you were using at least one of the monitors before, it's likely not a compatibility issue.
Just in case, I would connect an external monitor (even though it appears unusable) and then check for software updates in the App Store - maybe there's a necessary firmware update to install that has yet to be triggered.
What I would do next is to test the issue in a new user account - if it happens there, it's a system-wide issue. If it doesn't, the problem could be with a corrupt system preference file somewhere in your user library that needs to be regenerated. If you have a backup from before this issue occurred, restoring your user's home folder from the backup would actually be the quickest and easiest solution to this problem, as hunting down individual preference files can be tedious when you don't know the exact one causing the issue.
If it's a system-wide issue, it's likely OS related not system library .plist related as you still had the same issue in safe mode. (Although if the monitors are identical it could be a driver issue and I would check the manufacturer's website for new drivers, but if they're different and both are not working that is highly unlikely.)
Next I would back up your system and then reinstall the OS from the App Store to see if that fixes it. I know that's time consuming but I usually set mine up before I go to bed and it's done long before I wake up.
Last but not least is an Erase and Install from the Recovery Partition, but let's hope it doesn't come to that!