MacBook won't detect external display

Try power down , unplug, with 15 sec, replug, all devices. Turn on all devices, Then on Mac boot reset NVRAM & PRAM by holding down cmd/option/p/r simultaneously as soon as you hear the startup sound. Hold them until you hear it a second time. Once you sign in, both monitors should work as before. This worked for me.

Hello, I did the SMC reset and now both external monitors work fine. I think that the problem appears when the system actually starts hibernating and stops receiving signal from one of the monitors (as in my case I use 2 external ones).

This may sound ridiculous but I solved the problem with my work Macbook Pro 2017 13inch running High Sierra 10.13.6 by simply changing the adapter for the monitors Mini DisplayPort cable from an £ 49 Apple Thunderbolt 2 - Thunderbolt 3/USBC and downgrading it to just a £8.00 Mini DisplayPort to Thunderbolt 3/USBC from Amazon.

I did a restart and then had to Detect the display but it worked instantly.

Maybe High Sierra has trouble detecting the display information when it is being transferred along a Thunderbolt two profile not sure.

Hope this helps.

Have you done an SMC reset yet? This should resolve it. It did for me: High Sierra: external monitor not recognized since update ...

Well I had the same problem connecting a 2017 iMac to a secondary screen and have tried pretty much everything from fresh installs of High Sierra, VRAM etc... Downgrading to Sierra will solve your problem and I even thought about buying a USB-C monitor (until I saw the price!!) but eventually as a last resort I bought the Apple USB-C to Digital AV adaptor and this has solved by problems and things work now. Just one point though, I originally purchased a third party one, but did not work at all. In the UK the adaptor is £70 from Apple, but was able to pick it up for £45 from Amazon.

One cause might be that the 2016 MacBook Pro uses USB 3.1 over USB-C, whereas the 2017 model uses Thunderbolt 3 over USB-C. You might have better luck with a Thunderbolt 3 to DisplayPort adapter if you can find one.

I've had a problem that I think is related, in that I can't connect to an FTDI device from my 2017 MBPtb with an adapter, but I can connect to it just fine under the same OS with a 2012 MBPr. I suspect there's something going a bit wrong with the conversion from Thunderbolt 3 to USB.

I had tried everything suggested in this discussion. Finally, I changed the cord I was using for my monitor from an HDMI-lightening cord, for which I was using an Apple lightening to USB-C adapter TO an HDMI-HDMI cord with an Apple HDMI to USB-C adapter. It seemed counter intuitive, but it finally works now. After two months of screwing around, it came down to the cord. (The old cord still works with my older macbook pro.) For me, it just came down to the right cord for my monitor. I would experiment, (save your receipts) and find the right one for you. Good luck!

I've been having the same issue with both my iMac and my Mac Book pro. They no longer connect to a monitor that I have been using for years, but my fiance's windows computer can, so I know it's not the monitor or the cord. I use a lightening adaptor to whatever the standard monitor cord is with the pokey pins (as you can tell I'm really techy). My monitor is a hella old (technical term) Dell monitor.

I tried all the suggestions including downloading the display link driver and that didn't work. Tried resetting SMC (while disconnected from the monitor) and that didn't work.

HOWEVER, user Pooh1952's suggestion did work (thanks!):

With the monitor plugged in, I turned the computer off, then back on and reset NVRAM & PRAM by holding cmd/opt/p/r when I heard the startup sound until I heard it again. Logged in and the display worked!

BUT, when I unplug the display and plug it back in, it stops working. I only need it for my iMac though so I should be able to just keep it plugged in. It stays connected when it's asleep but I haven't tried restarting it yet... <:\

Best of luck everyone! This is stupid. 😀

I have had this same issue with an LG ultra wide 34" monitor and my MacBook Pro (2017). It would work only periodically with the connection from Thunderbolt 3 to the thunderbolt connection on the back of the monitor. (It worked perfectly until the High Sierra update) It has been over a month of back and forth with apple and LG. The apple engineers recommended using a thunderbolt 3 to display port cable. It now recognizes the display and has been working for 3 days. Unfortunately, the display is not as sharp as the thunderbolt connection was. The thunderbolt 3 to HDMI cable also allowed my MacBook to be visible but the resolution was really bad. I wish an update would come that would allow me to connect like I was prior to High Sierra.

Hey folks,

Came across this post as I was having the same problems. Reset my SMC as suggested above and now everything is fine. I was also having intermittent battery issues that have been resolved.

I had the same problem on an iMac 5k. Resetting NVRAM resolved it.

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Shut down your Mac, then turn it on and immediately hold down these four keys together: Option, Command, P, and R. Keep holding the keys for about 20 seconds, during which your Mac might appear to restart.

- from How to reset NVRAM on your Mac - Apple Support

Someone posted this in another discussion last night and it's been working for people with iMacs and MacBook: http://www.displaylink.com/downloads/macos

Here's the post: Re: External monitor problem iMac 2017 High Sierra.

SMC reset did end up working for me though. I'm just running two monitors via adapters in the thunderbolt ports.

I had same effect on my Mac mini when upgrading to High Sierra. The software does not look for an external monitor. My fix is simply when Mac is up and running simply disconnect cable, then reconnecting cues software to see external monitor and sends signal to monitor.

I installed High Sierra on my Macbook Pro and then my mini display-port to HDMI stopped working. I tried every reset and different HDMI cables, nothing worked. Then I found the problem. When installing major updates on Mac's if you leave devices plugged in, as the computer restarts several times during install power surges through your ports can knock out your external devices. Well that's what happen, it broke my mini displayport adapter. after using a new one, problem solved. High Sierra or Sierra can knockout Thunderbolt docks, display adapters or anything connected to your computer. So next time, unplug everything connected to your Mac before updating.

Okay. Full disclosure (even though I tried this a couple times before) it seems unplugging the monitor—at the source (not just from the back of the monitor) for 15+ secs and then plugging back in cleared up the issue. 😐

My 2017, 15” MacBook Pro is now connected via third-party mini display port to usb-c adapter, to my 27” Apple Display and working perfectly as I type this. Crisis averted, for me anyway. Good luck everybody.