TBT3-UDZ issue with second display

I just purchased the TBT3-UDZ for use with my Macbook Pro 16inch runnint Catalina 10.15.6. I can’t seem to get the second display to work. It blinks as if it’s detecting a signal but turns off again. Both monitors are plugged in using HDMI. I’ve confirmed both monitors work when plugged in individually.

Thanks in advance.


Thank you for contacting Plugable support! Sorry to hear about this issue. I’d be more than happy to assist you.

I have a few questions to try and find out what may be causing this behavior:

  1. Are you using the provided Thunderbolt 3 cable that the dock shipped with to connect it to your MacBook Pro?

  2. Are both monitors the same brand and model? Or are you using different model displays? And, what brand/model display(s) do you have?

  3. If you try swapping around the monitors on the two HDMI ports on the rear of the dock, does the issue stay with the HDMI port on the dock, or does it follow the monitor over to the other port?

Please don’t hesitate to let us know of other questions.

Thanks again for contacting Plugable support and best wishes!

Joshua Henry

Senior Engineer | Product Owner
Plugable Technologies

Thanks Joshua. Here’s the answers to your questions.

  1. The cable provided wasn’t long enough so I used one that I had. I didn’t really think about the cable not being Thunderbolt. I’ll check that out shortly.
  2. Monitors are not the same. One is an older NEC multisync ea244wmi. The other is a new LG (I’ll get the model shortly).
  3. I tried swapping the monitors and the issue stays with the second HDMI port. So both monitors will work on the first port.




I went back and tried the cable that came with the device and it works as expected. I should have checked the other cable before using it. Thank you for the suggestion.


Thanks for the updates! Glad to hear it is working with the provided cable.

Thunderbolt 3 docks are very demanding of the cable being used. If you need a longer cable, you’ll want to ensure it is rated for Thunderbolt 3 at 40Gbps. Standard USB-C cables won’t work correctly with this dock on Mac and will limit the dock’s ability to function correctly.

While this dock can also operate in USB-C mode, it is not ideal, and Mac systems have more limitations in C mode versus a Windows system. The dock in this mode on Mac would have been trying to mirror the first display to the second (as Mac does not support extended displays over C mode) and since the displays are different models, I suspect it was not able to properly mirror the first to the second.

You should be able to find a longer Thunderbolt 3 40Gbps cable if needed, many manufacturers (including Apple) make up to two meters in length. At this time, our longest 40Gbps cable is the 0.8m cable the dock shipped with.

Should you run into further issues or have additional questions, just let me know!

Thanks and best wishes!

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