External monitor can't be detected anymore

Question

I know this is a reoccurring issue and I've gone through every forum, every topic and every blog post I could find. Two days ago my MacBook Pro 13 stopped detecting external monitors.

I'm using a Unitek HDMI to USB-C dongle which I know works fine. I've tested 3 different monitors, that have different resolutions and refresh rates, they all work fine.

I've got another 2019 MacBook Pro 13" that works fine with every single monitor; using the same cable/dongle combo.

What I've tried:

• plugged the dongle with and without charger also being plugged in,
• plugged the dongle when Mac was sleeping/off
• plugged in a USB-C -> USB-A dongle and connected a mouse to it - both ports work fine,
• restarted my Mac after I've plugged the USB-C cable in,
• reset NVRAM and SMC,
• reinstalled OS (with and without backup)
• tried clicking "Detect Displays" in Displays settings,
• closed the lid, plugged the cable in, opened the lid,
• installed DisplayLink drivers, uninstalled them.

When I connect the dongle, it's visible as BillBoard Device in the System Information -> USB (both Macs have that). I've also been watching and comparing Console logs when I connect/disconnect the monitor and here are some things that I've noticed:

• The Mac that works fine has some [Hot_Plug] events visible when I plug in the monitor,
• FB0 both have DisplayPort driver version 1.1, FB1 on working Mac has a version 1.2 (this information doesn't show up on non-working Mac)

The screen doesn't flicker when I connect any external display, there's no Arrangements tab visible. AirPlay works fine.

Both Macs are 2019 13" Pro with the same stuff inside - they both use Intel Iris 645 Graphics.

Any tips?

Answer 1

There's only one thing to do.... take it in for service.

When I connect the dongle it's visible as BillBoard Device in the System Information -> USB (both Macs have that).

The fact that it's being detected means your port is functioning. Now, you didn't specifically say so, but I'm assuming that the built-in display is working. Since you have the Intel Iris (Integrated) graphics, there's no failure with the graphics chipset (there's no discrete GPU)

However, there is a video mux (multiplexer) chip that "sends" video to the LCD, the external monitor, or both. This has likely died.

• Macbook Pro on but display is blank

You've done a lot of diagnostics - actually gone a bit overboard with the re-installation of the OS. Safe Mode or Recovery Mode is sufficient for this. For the future, try this first and if you absolutely must have a clean install, do it to a USB flash drive (32GB is sufficient) so you're not wiping out your drive (and all your work) as a diagnostic procedure.