External display not working after installing MacOs Catalina

Hi, I had the same problem after upgrading to a Macbook pro 2018. The 2nd monitor would take a long time to be recognised. Random timeframes. Sometimes it was recognised instantly after plugging the HDMI>USB C cable, but sometimes it could take 30 minutes.

After trawling forums I found it could be some kind of frequency that different cables have. Newer cables it seems have different frequency to older ones. However none of the cables spec this frequency in the tech details so you can't easily check.

In the end I just bit the bullet and gambled - bought a new HDMI>USB C and plugged it in.

Voila.

I've been running the new cable for about 4 months and have had 100% uptime. Not a whiff of issues (touch wood). So anyone with this issue who is a bit lazy like me and just want to try a few things could try this. For a relatively small amount of cash it may sort your problem.

Good luck.

Using Cleanmymac is not the solution for anything. It is like putting the fox in charge of the hen house.

Ut has been reported by several people that a change of cable and/or adapter is often what it takes. Apparently Catalina (or, I’d guess, an updated firmware that may be installed by it) is far more sensitive to precise voltages in the cabling.

For now, this is fixed. Here’s what I did:

Started chat session with Apple tech, wasn’t charged.

First I was asked to upgraded to the newest OS. I was using a VGA to thunderbolt C adapter, but I did try HDMI and that also did not work.

Together we reset the SMC together. Now this is the clincher: the method of doing this is different for many models, and I would never have gotten this right if I didn’t have an Apple tech on the phone.

Safe restart. (shift-restart) We didn’t reset the PRAM on this call, but I’m confident my attempt to do that went thought.

The VGA connection was still problematic but the HDMI worked immediately.

So now I’m using HDMI only.

The tech people were not aware of this an an issue.

If using an external display attached to a notebook computer with a built-in display, ue

system preferences > Displays ...

hold down the option key and the (Detect Displays) button should appear in the lower right corner, and clicking it will send the query to all potential display ports.

Next best is to unplug/count to five/replug the cable DIRECTLY on the Mac.

I was having the same annoying problem - tried a lot of the solutions suggested here. In the end a new USB-C to HDMI cable did the trick for me. I used the 60hz StarTech.com cable. Connected a Macbook Pro 2018 and a Philips 243V monitor. Has been working all day for me now. Hooray!

The "huge problem" is using adapters that need specific third party software ("displaylink") to work with macOS.

Each time you update macOS you have to download or wait for a new driver.

There are plenty of adapters that do not require extra software and continue to work just fine.

I updated my 2019 MacBook Air to the Catalina OS 10.15.6 a few days ago and had the same problem as everyone else in this thread: my 2019 Samsung monitor wouldn't no longer work. I felt a sense of dread as I went through all 11 pages of the questions and advice; my questionable-tech-savvy brain could only process about 50% of the advice. After trying a handful of the suggestions, none of which worked, I tried this today: (1) I powered off the monitor; (2) I disconnected the monitor from my laptop; (3) I reconnected the monitor to my laptop; (4) I turned on the monitor. It worked! I have no idea why. It seems like different solutions are working for people - maybe it's due to different Macs and different set-ups? My fingers are crossed that this solutions will "stick" and I hope that others will find a solution that works for them.

What is apparent is that there is no single solution. My issue I did not see the monitor in the Arrangement space. It just did not see it at all. I did a PRAM reset as suggested and it did not work. I have one monitor using a DVI to HDMI cable and the one that will not connect using a DVI to USB-C connector. Today I changed it to use the same type DVI to HDMI cable (and adapter to hook into USB-C slot) and it works ... at least for today. Regardless, Apple has a bug here they need to fix.

I had problems with my HDMI to USB C cable connecting a VIZIO external monitor to an iMAC a month ago. The monitor was intermittently detected after I rebooted. Contacted VIZIo, but they don't do Mac OS drivers. I had tried all the fixed mentioned earlier in the thread, and I ended up buying a USB C to VGA cable. I have not had any problems since.

Maybe SideCar broke it! I have an odd workaround.

I had the same problem: external ViewSonic was working fine (w/ DVI to USBC converter), but it stopped detecting that external display after Catalina upgrade. Also tried WebEx which refuses to share my screen now, only shows background.

My "dead" display was connected when I tried out SideCar (extending my display to my iPad Air). Once I did, not only did the iPad extend my Mac desktop, but that ViewSonic external display came online. I haven't determined yet if it's a permanent fix or if I need to toggle SideCar each time to get my display to connect.

I had similar issue with mid 2014 Retina MacBook Pro. Apple Support recommended reset PRAM etc resets but what worked for me was disconnecting the cable from my external monitor (Samsung) and then selecting input source on external monitor (DisplayPort in my case). I then reconnected the cable to my MBP's Thunderbolt port and it worked OK.

I'm the one who was one of the first to post this issue in this thread and got the snarky reply about CleanMyMac (still no issues). However, it was not foolproof. When shutting down my computer, sometimes it would still not recognize the one of two monitors with the HDMI to USB-C connection. After further research, I think it does have something to do with the HDMI. My 27" Viewsonic also had a VGA connection as well. I bought a different cable (USB-C to VGA) and never had another problem.

I updated my macbook air to catalina last night. Tonight I went to go plug in my external monitor and it wouldn't recognize even after trying to detect. I usually have my monitor on even when it's not connected to my mac, so I decided to turn off my monitor and turn it back on, while connected to my mac and the screen came up.

This was *THE MOST* frustrating problem I have ever had with an apple device and/or software update. It's been dogging me for months, but I *FINALLY* figured it out.

As background, after the Catalina update in November, my MBP which had been used to drive 2 external monitors stopped detecting these monitors. No change in monitors. No change in cables. No change in hardware of any type. It worked flawlessly for 2 years pre-Catalina. After a few minutes randomly, the screen would flicker and one monitor would be back on. I would work through the day, sometimes minutes later, sometimes hours later, the 2nd one would flicker and then come on. If I ever disconnected from the monitors or -- god forbid -- had to restart the MBP, I would be monitor-less for an indeterminate amount of time.

Before I get to the solution, here is what didn't work. I searched the forums for months:

1) Resetting NVRAM , PRAM, or SMC.

2) Restarting the MBP

3) Unplugging and replugging

4) Power cycling the monitors

5) Updating drivers

6) Updating to latest version of OS

7) Buying a new-ish monitor (the old ones were... well, old)

8) Using a new USB C Hub

Apple certainly never admits to these types of problems, and doesn't let the public know when stuff is broken. Like I said, it was frustrating.

So, the solution for me was the cable. I looked up the 2 year old cables, which were HDMI to USB C (purchased when HDMI's were removed from MBP's). Fortunately, Amazon had the order history and I was able to see that what I had were 4k 30HZ cables. When I replaced with 4k 60 Hz cables, it finally -- BLESSEDLY -- worked.

This is unconscionable for Apple to break these and then neither admit to, nor respond to the many forum posts that requested help with this.

So, buy new cables that support 4k 60hz, *NOT* 30hz. I got a J5create 6 foot HDMI to USB C cable for $30 from Best Buy, but I can see that there are cheaper options on Amazon.

Just an update on this folks...these cables seem to be working much better now. It took my laptop a day or two to adjust to them; but now they are working every-time I start my laptop. There is still the occasional problem of one of the monitors going out, and its usually the same one, when the monitor wakes up after being asleep for a while; but either an adjustment on the monitor by cycling through the various settings or a reboot does the trick. Again, this is very rare that this happens at all...very occasional. These are the cables I'm using: https://www.amazon.com/gp/product/B075V5JK36/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1