External monitor problem iMac 2017 High Sierra.

I have the same problem with a TV.

The monitor worked for some time, but in an instant it turned black and now I can't get it to display anything. The Macbook detects the TV in the monitors section of the Preferences panel but the TV shows "No signal".

I got the problem fixed momentarily by reinstalling Mac OS, but then it happened again.

When my Mac is connected via HDMI to the TV by mouse usually hangs for a second or two every few seconds.

I don't have this problem with an HP monitor I'm using, at least it didn't happen until now.

I had a chat with Apple last evening up to a senior advisor. Officially, they are not aware of any problem.... Besides, they don't look or trust the Apple Community discussions, which I asked them to. Finally, I hope I managed to make them understand that there are quite a lot of professionals and loyal customers out there who are getting upset because of not being able to do their job. They won't acknowledge that High Sierra s...cks for display connections, but I hope that the message will reach their brain.

However, they suggested to reset the VRAM, which could be a good idea (for those who don't remember, you restart while hitting alt-ctrl-P-R, and the system must do a "bong" sound, before you can release it. That will empty your video ram. I haven't tried yet (today my second display works... for now), but I wish I could try it and get some feedback from you guys, if you can try and see if it can help (at least until they fix the problem).

Running two external screens worked fine on my MacBook Air on last version of OS. Installed High Sierra and now only one external monitor works. It works using display port to HDMI adaptor, but a second one will not work using USB to HDMI adaptor. Wasn't a problem before installing High Sierra.

Resetting the VRAM did the trick for me too. I got my two monitors back. Now I’m noticing the fan going on nonstop when the two monitors are attached. It stops when I unplug them and use the laptop screen but, hey, it’s better than nothing.

I have this issue and troubleshot mine down to some specific items. I have a 2017 15" MacBook Pro and everything worked fine until High Sierra. I have two external monitors that I could not get to work anymore.

What I have now discovered is that if I use a multi-device adapter (e.g. USB-C to HDMI and USB made by Apple), the monitor will not work at all; however, if I use a USB-C to HDMI standalone adapter, I can get each monitor to work. The other caveat is that I have to plug the adapter in "right side up". USB-C isn't supposed to have a "right side up", but if I plug the adapter in upside down it stops working.

I have tried two different multi-device adapters (one made by Apple and one made by Tripp-Lite) and neither work. I have tried two different HDMI standalone adapters (one made by Tripp-Lite and one made by CableCreation) and both work, but only "right side up".

I tried everything else in this thread with no success, but now I have dual monitors working. I hope this may help others.

Biotherapeutics,

I am using a late 2013 27" iMac and tried the NVRAM reset a couple times without any success. "cmd-alt-p-r" on startup waiting to hear opening sound a second time a bit louder. I then connect ext. monitor via "USB-C - HDMI" "thunderbolt - HDMI" adapter and then the iMac goes blank with the mouse pointer. If I unplug ext. monitor from power source I get iMac display back.

I've tried the reset a couple of times now. Not having my ext. monitor is costing time and I'm about to go back to Sierra again through time machine because this extra monitor is nothing more than a paper weight sense the High Sierra upgrade.

Any suggestions appreciated =-)

Resetting the VRAM doesn't work for me also, tried a couple of times.

I could get it to work when I update to the stable build or from the stable build to the current beta (which I'm in at the moment). But after a while it stops working again. This might mean that something during the macOs builds install process makes it work again, like some cache being cleared. Already tried going into safe mode and then back and no luck.

As I've said before, installing a new build solves the problem for me. I've installed the new beta that came out yesterday and that fixed it but I still don't know if it's a momentary fix or if the issue has been really fixed by Apple.

I'll check on the next few days if the issue comes up again.

Similar issue here. Got a Macbook Pro 2017 13-inch yesterday with an Apple multiport Thunderbolt adapter (HDMI, USB, USB-C). The external Dell display turns black arbitrarily and I have to re-plug the HDMI several times to make it work again until it happens again in 10 minutes or hours. I've spend the whole day trying everything mentioned in this thread, like NVRAM, SMC, DisplayLink, reinstalling the OS. Nothing helps.

This not quite the premium experience I expected.

Same issue here with 10.13.1 on a late 2013 MacPro. Using a second monitor is unreliable. Sometimes, even plugging my Thunderbolt backup drive into the computer shuts down the monitor. The monitors are also unreliable when waking from sleep. They just stay blank and it appears that the machine has crashed, but I'm pretty sure the Thunderbolt bus is just taking a powder. It doesn't really matter, a reboot is still required.

Apple really messed this one up. High Sierra (even 10.13.1) seems unstable for anything other than an All-In-One Macintosh without a second monitor. I wish I could revert to Sierra, but with the filesystem change, that's really not an option.

Apple should have discovered these problems in testing, but the reality is that their focus and resources are mostly dedicated to the iPhone and iOS.

I have been running successfully 2 external displays (Samsung S27C750) for over a year from a 13" MacBook Air: 1 linked via USB to VGA adaptor and the other from the standard DisplayPort with HDMI adaptor.

I have then added a Thunderbolt dock in-between, running for 3 months without any problem.

I just upgraded from Sierra to High Sierra... The USB to VGA is not working anymore! I wish I did not upgrade!!! I now lost half of my desktop!

Weird suggestion (I hope temporary until Apple solves it) that worked for me is to disconnect HDMI from adapter, connecting it again and at the same time move and click the mouse/trackpad in the external desktop monitor. Somehow like "activating it".

As I said... weird but keeps me working on my external monitor.

Cheers!

Same issue on Mac Mini - I used to have three monitors hooked up to my Mac Mini - two through Thunderbolt, and one through a USB to HDMI Adaptor (Coredy MDA 1920). Worked Great based on this: thunderbolt - Hook up 3 monitors to Mac Mini Late 2014 - Ask Different).

Talked to Apple - reset NVRAM, that didn't work, so all they could say after that was that the mac mini only allows two monitors and there is nothing they can do about it.

I'm looking forward to a solution!

I have a MacBook Air and up until updating to High Sierra I was successfully using two external monitors without any problem. Really frustrating that after updating I go from 3 monitors at work to 1! I am trying to downgrade back to Sierra to see if that resolves the problem. However, Apple really needs to fix this STAT!

I'm having the same problem since I upgraded to High Sierra with a Samsung monitor(TV). I have to unplug the HDMI then reinsert it to get it to work.High Sierra doesn't remember all of my monitor settings on startup. Very Funky! I hope Apple is on this.