MacPro (late 2013) thunderbolt monitor not booting consistently

IIRC the Thunderbolt ports on this system are tied together in pairs. The top two ports on the left are related to each other and the top two ports on the right are related. The bottom most ports left & right are related. If you are having problems, then make sure nothing is connected to the related Thunderbolt port. Maybe the other device is interfering with the Display. Also do you have anything connected to the Display's built-in ports? If so, maybe try disconnecting those items.

Perhaps your display is having power problems. Try connecting one of the Display's Thunderbolt ports directly to the Mac Pro with a Thunderbolt cable which will help to provide extra power if the monitor has a power issue. This helped with one of our organization's Thunderbolt Displays and is an actual Apple troubleshooting step.

It doesn't help this Mac Pro has known GPU issues and I've found Apple's displays to sometimes behave erratically. Not a good mix.

to get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display.

This query is only sent at certain times:

• at startup

• at wake from sleep

• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go

• on invoking Option-(Detect Display) button in Displays preferences

so try doing some of those things and see if the display comes alive.

Modern Displays with multiple ports are sometimes busy scanning the other ports, looking for an input, and miss the query from the Mac. They need to pay attention to the port you are actually using, or they will miss the query.

Some displays have On-Screen Display settings that can be used to tell the display a computer is attached on a certain port, or a certain port should be highest priority. Changing those may make your display more responsive.

Some displays include their own private "sleep" settings for the display alone. This can allow the display to enter its own sleep mode, on top of the Mac's not sending it data. A display that is sleeping on its own cannot respond to the Mac's query, and will stay dark.

No.

The four bullet points in my response above are four 'handles' to get your Mac to create a new query. If this is your only display, the most useful may be to sleep and wake your Mac:

command-Option-Power or Command-Option-MediaEject to sleep.

any key to wake.

You could also try unplugging/replugging the cable -- the entire adapter and cable end where it plugs into the computer.

.

@Grant thanks for the illustration to make the layout understandable.

You're welcome. Hopefully this will continue to work.