Thunderbolt cable that only works as USB 3.1 I have an LG 5k Ultrafine screen which does not work when connected to my Macbook Pro 2018. Before thinking that the screen may not be working, it could be that the cable which isn't working.
In order for it to work, it must be a Thunderbolt 3 cable, which I know it is (I bought it and it used to work with the screen). I plugged the cable Macbook &lt;&gt; Different USB-C device and it did work, however it Apple -&gt; About this Mac -&gt; System Report, the device only gets shown in the USB 3.1 bus instead of the Thunderbolt section.
Are there ways to test if the cable that I have is a working Thunderbolt 3 cable?
I'll step through some obvious reasons this might not work, so don't feel offended if this sounds &quot;basic&quot; and you've done this already.  The setup you describe should work so it comes down to a bad setup or bad hardware.
The first thing is to see if the cables are in fact Thunderbolt cables.  They should have USB-C connectors on each end and the Thunderbolt icon on one or both connectors.  If you don't know what the icons look like for the different connectors/ports then take a look at Apple's support page on this: https://support.apple.com/en-us/HT201736
The second thing to check is that the devices needing the most bandwidth are connected closest to the computer.  Connect the Thunderbolt cable to the Thunderbolt ports on the Mac and the display.  Any USB-C devices connected in this chain should be connected to the ports on the back of the display.  It's best to connect any USB-C devices directly to the Mac if you can since they can &quot;steal&quot; needed bandwidth from the display.
A Thunderbolt cable is &quot;dumb&quot;, as in these cables don't often have any electronics in them to identify them as a Thunderbolt cable when connected to a Mac.  A Thunderbolt cable when used to connect a USB 3.x device will look like a USB cable to the computer, it doesn't know the difference.
If walking through the wiring and checking that it's like I describe then perhaps something is broken.  Do what you can to verify all cables are working, as well as the display and devices.