MacBook won't detect external display

Got mine to work plugging it in to a HDMI port, a little set up, and back to my DVI monitor...

(my adaptor has dvi, vga, hdmi and a usb, if yours is similar)...

I just got a new 15" MBP this weekend and my adaptor arrived today, I tried connecting a DVI external display (Dell UH2311H) and nothing was happening. The display would detect connection then just enter sleep mode, the MBP would carry on as if nothing happened. Tried restarting, nvram, shut down/turn on with adaptor connected/not connected etc, still no joy.

My adaptor also has USB, VGA and HDMI adaptor ports, so I took the MBP to my TV to try the HDMI, and it got signal and worked. I then proceeded to set up the displays a little, resolution for the TV, turned off mirroring display 'use as separate display' then disconnected the HDMI, went back to my desk and tried my monitor again and it worked!

Hopefully this'll be helpful to others.

I was facing the same issue... I did NVRAM reset and it did not work... then I did SMC reset and it worked for me... But it was annoying to do that... Apple, please don’t do this to non technologically advanced people... Give the permanent solution...

I HAVE FOUND THE FIX!

After I updated to the latest High Sierra my displays would no longer be recognized.

I tried all of these and they all failed:

1. Restarting computer
2. Reseting PRAM
3. Checking all cables
4. Installing all updates

These WILL NOT WORK.

Through this update apple has removed the ability to use docking stations that are not specifically designed for MAC and/or are old technology. Meaning adaptors/docking stations that require any third party drivers such as Display Link and DisplayPort Adaptors.

The Fix-- you can still use your monitors with a USB-C to HDMI adaptor. You can no longer use dell docking stations or adaptors that are not "Apple/Mac Friendly"

The SMC reset fixed this issue on my MacBook right away after the update. Super quick fix!

Try resetting your SMC

screenshot taken from http://osxdaily.com/2010/03/24/when-and-how-to-reset-your-mac-system-management- controller-smc/

Download the very latest OS update. This finally fixed my monitor issues.

I had the same issue and with the SMC reste now the screen is back up and running. Just wondering if this will be a sort of routine operation or Apple will fix it with the next SW release.

I had the same problem so I looked up how to SMC reset my mac and it fixed my problem. So maybe that might help

Just spoke with Apple enterprise support on this. He was very helpful! It is not a High Sierra issue, in my case I needed you upgrade the driver of my external device. I am using a diamond DisplayLink and I updated the firmware and it now works perfectly. Hope this helps!

I am using MacBook Pro 2017 and had same issues while connecting external monitor. I did SMC reset today. Right after the SMC reset, the issues were not resolved. Even I could't access wifi (especially, guest wifi) while connecting with the external monitor.

Several hours later, when MacBook is on sleep mode, I tried to connect with the external monitor and then I waked the MacBook up. Suddenly, the issue was resolved. I can use dual-monitor now. I am using two connectors: miniDP(RGB-HDMI) and a connector (HDMI-USB C). Hope it is helpful for you.

In summary, do SMC reset --> connect with external monitor when MacBook is on sleep mode --> wake it up.

I had the same problem, the macbook pro 2017 would not recognize the HDMI 27" monitor (brand new & high res) when connecting it through a number of converters or a multiport C connected hub with an HDMI output. I decided to simplify and bought a USC-C to HDMI converter cable from DDUP on Amazon for $20.

(DDup USB C to HDMI Cable 4K 60Hz, USB 3.1 Type-C to HDMI Cable for Macbook/Macbook Pro/ iMac, Samsung Galaxy S8/S8+/Note 8, ChromeBook Pixel/Pixelbook, Surface Book 2, LG V30, Thunderbolt3 - 6 Feet).

I plugged it in on the right had side c port (where the mini DVI display port was in past generations) and it popped right up - auto recognizing it.

Also - if you want a terrific, affordable 27 inch monitor for your new macbook pro, I highly recommend the

ASUS 27" WQHD (2560 X 1440) IPS DP HDMI VGA Eye Care Monitor 27-Inch Screen LED-lit Monitor (VZ27AQ) from Amazon for $320.

It looks as sharp and clear as the Mac's retina display, very impressive, at a reasonable price.

What I'm saying is, when all else fails and you've tried everything recommended, then replace adapter or cable that you use. I had a friend with an extra adapter and didn't have to buy one. But of course, this can only be the problem if your external devices were connected during upgrade, if not connected at the time then maybe a software issue is the problem. I'm don't needing to try anything else because replacing my adapter worked. Someone suggested that my adapter went bad on it's own and I've seen that happen before but at the exact time of update, I don't think so. The High Sierra security update is to prevent a flaw in the Intel chip set that allows someone to meltdown your chip set or burn up your battery so maybe this update can surge power settings through your connected devices. It best to disconnect everything before upgrading and then you know for sure the upgrade cannot damage your external devices. There's to many people posting trying all other solutions and nothing helping. This is just one more suggestion, when all else doesn't work. It worked for me. It's harder for people with Thunderbolt hub's because they cost so much.

FWIW, I'm getting reasonable consistency with the following procedure, identical to many suggestions on this thread:

1. Restart MBP

2. Plug in dongle

3. Plug in HDMI

(my interpretation: Give the dongle drivers a few seconds to load, so HDMI will encounter a working interface)

Seems to be okay if I put MBP to sleep:

1. Put MBP to sleep.

2. Turn everything off

To re-start:

1. Turn everything back on

2. Bring MBP out of sleep.

Your mileage may vary!

I contacted both @DellCares and @AppleSupport for the same issue...

One of the tech guys suggested that perhaps the monitor needs a firmware update to be compatible with the latest version of macOS High Sierra 10.13.3?

I haven't tried this but I found this article by display link: https://support.displaylink.com/knowledgebase/articles/1188004-macos-high-sierra -video-functionality-not-enabled

Basically, "macOS High Sierra 10.13, the operating system applies a stricter security policy to software that requires kernel extensions (kexts). The DisplayLink driver installation includes a system extension (DisplayLinkDriver.kext) that is affected by the new policy. As a result, the user now has to explicitly enable this system extension in the Security & Privacy system preference after installation and before the DisplayLink software is allowed to run."

Give that a try?