MacBook Pro 14 external monitors keep switching positions

Hello, trooters.

To start off with, this article gives great information and tips on using multiple external monitors: Use one or more external displays with your Mac - Apple Support.

Beyond what you have already done to troubleshoot this, we advise that you continue by testing to see when this behavior occurs. First, put your Mac into safe mode and test. Booting into safe mode will disable any startup and login items from automatically opening, which can help us to isolate the source of the issue further. When you do this, you’ll see some unusual behavior from the display; this is to be expected. Please follow the steps in this article: How to use safe mode on your Mac. 

Next, let's see if this behavior persists in a second user account on your Mac. Please follow these steps to make a test account that can be deleted later: Set up users, guests, and groups on Mac.

Let us know how this goes, please. Thanks and have a fabulous day!

To determine display names and capabilities, you Mac uses something resembling "plug-and-play". it sends a query to the display, and the display responds with its name, its capabilities, including default and preferred resolutions, and its SERIAL NUMBER. The serial number is how the Mac uniquely tells TWIN displays apart.

<< The monitors I have are ASUS PA248QV 24" monitors (both the same). >>

What we have seen here with TWIN displays, sometimes the manufacturers have been sloppy about what field contains the serial number. The Mac uses VESA standard, and if the display-maker does not supply the serial number in the expected field, the Mac can not tell the TWIN displays apart, and they 'swap around'.

Your displays have a design defect. only the display maker can fix this defect, and would have to issue a display firmware update to fix it. you should attempt to pressure them to fix the defect.

as a work around, two different model or different makers will not exhibit this symptom.

additional background information:

the details can be seen using a Utility like SwichResX, which reads out the EDID information the Mac gets back from a query of the device for its name, capabilities, and serial number. in an earlier investigation a User found that the serial number was there, but NOT in the correct field, so the Mac could not tell TWIN displays apart.

Apple has not responded favorably to suggestions that they "accept whatever junk the display provides, whether it meets the accepted standard or not".

Thanks for this. I, too have 2 ASUS ProArt displays (PA278CVs), and my MacBook is getting confused as to which is which. According to the EDID information from SwitchResX (Thanks again, cool app), the serial numbers are different, but what is the correct field they should be in so macOS reads them correctly?

Under Displays in Settings, macOS labels them as PA278CV (1) and (2) respectively, but under System information it's

Displays:

PA278CV:

Resolution: 2560 x 1440 (QHD/WQHD - Wide Quad High Definition)

UI Looks like: 2560 x 1440 @ 60.00Hz

Main Display: Yes

Mirror: Off

Online: Yes

Rotation: Supported

PA278CV:

Resolution: 2560 x 1440 (QHD/WQHD - Wide Quad High Definition)

UI Looks like: 2560 x 1440 @ 60.00Hz

Mirror: Off

Online: Yes

Rotation: Supported

I'm trying to contact ASUS support to see what they say.

"Apple has not responded favorably to suggestions that they "accept whatever junk the display provides, whether it meets the accepted standard or not"."

Ironic, as I'm only using two external displays as the supplied macbookpro screen failed after 18 months. Junk indeed.

This issue is really going to determine whether my replacement machine will be a mac or not. Judging by other comments, event the mac mini, designed for use with external monitors can't cope with many of the monitors people are using around the world (by the way, many folks don't buy a new monitor every time they change PC. Nor should they have to).

Thanks trooters,

At this point, we recommend that you reach out to Apple directly for assistance with resolving this issue here:

Get Support

Cheers!