MacBook Pro M2 Max and (3) THREE LG Ultrafine 5Ks

RE: “By reading the mentioned recommendation, I should connect it physically to the HDMI port in my Macbook.”

You are not reading the specifications of the LG Ultrafine 5K monitor correctly. It requires Thunderbolt input. You cannot connect it via HDMI. The fact that a Mac has a HDMI output does not mean that you can connect any non-HDMI monitor of your choice to that HDMI port and expect it to work.

Problem identified!!! So here's the deal: The M2Max MacBook Pro (and I don't know what other models) will not drive (2) LG Ultrafine monitors with the same model numbers (At least the two pairs I tried).

I tried (2) 27" 2020 LG Ultrafine 27MD5KL-B and they would not work together with nothing else connected.

I tried (2) 27" 2018 LG Ultrafine 27MD5KA-B and they would not work together with nothing else connected.

I could run (1) 27" 2020 LG Ultrafine 27MD5KL-B (1) 27" 2018 LG Ultrafine 27MD5KA-B and (1) 24" 2022 LG Ultrafine 24MD4KL-B with no issues all driven off Thunderbolt connections and could even drive a TV on the HDMI port as well and use the laptop display all at the same time and manage the screens just like we all would expect.

Now I am running (1) 27" 2020 LG Ultrafine 27MD5KL-B (1) 27" 2018 LG Ultrafine 27MD5KA-B and (1) 27" 2021 LG UltraGear 27GP83B-B (connected with a USB-C to HDMI adapter) as well as a TV on the HDMI port and can use the laptop display all at the same time and manage the screens just like we all would expect.

It looks like a duplication issue that won't allow two LG monitors of the same model number to run at the same time. At home, I do run two identical Dell monitors at the same time without issue so the problem appears to be limited to LG monitor (perhaps just UltraFine) drivers.

Note: The Sonnet Echo 11 Thunderbolt 4 Dock will also drive 2 monitors when connecting 2 different model LG monitors to it. It will not drive all 3 monitors through the (3) Thunderbolt 4 outputs but I am pretty sure the literature even says that.

The cables I am using are nothing special and are working properly but I did try shorter cables that came with recent Thunderbolt 4 devices (1.5' or less) and they didn't change the outcome of these tests.

I hope this is of some help.

I have the exact same issue - (3) LG Ultrafine 5Ks and a 16" M2Max - I actually have (2) M2Max 16" MacBooks and they have the identical issue so it isn't a hardware problem.

Step 1: Connect LG monitor #1 via Thunderbolt port 1 on M2Max - Monitor 1 works.

Step 2: Connect LG monitor #2 via Thunderbolt port 2 on M2Max- Monitors 1 & 2 work.

Step 3: Connect LG monitor #3 via Thunderbolt port 3 on M2Max - Monitor 1 works - monitors 2 & 3 go dark but Monitor 2 still shows up in system settings as a recognized display.

Step 4: Disconnect monitor #2 from thunderbolt port 2 - Monitors 1 & 3 work.

*This happens whichever order you choose to connect in. 2 monitors will work over thunderbolt as long as you don't' have a monitor connected to the third thunderbolt connector. Problem is - Apple's specs show it can drive 3 thunderbolt monitors.

Having another monitor connected to the HDMI port has no bearing on the above results either way.

I am wondering if it will work with a Thunderbolt 4 dock which I will try tomorrow.

I also wonder if the standard LG Ultrafine color profile used in MacOS runs at a frequency above the MacBooks specs for 3 thunderbolt monitors so I will look closer at that tomorrow as well.

“M2 Max

Simultaneously supports full native resolution on the built-in display at 1 billion colors and:

Up to four external displays: Up to three external displays with 6K resolution at 60Hz over Thunderbolt and one external display with up to 4K resolution at 144Hz over HDMI

Up to three external displays: Up to two external displays with 6K resolution at 60Hz over Thunderbolt and one external display with up to 8K resolution at 60Hz or one external display with 4K resolution at 240Hz over HDMI”

In other words, if you connect a HDMI display with super-high resolution (8K) or a super-high refresh rate (240 Hz), that comes at the expense of one of the USB-C or Thunderbolt monitors you otherwise could have connected.

It does not mean that the only three-monitor configurations (for a MBP with the M2 Max chip) all require the use of HDMI.

If you have the M2 MAX and not just M2 Pro processor, it supports:

M2 Max

Simultaneously supports full native resolution on the built-in display at 1 billion colors and:

Up to four external displays: Up to three external displays with 6K resolution at 60Hz over Thunderbolt and one external display with up to 4K resolution at 144Hz over HDMI

Up to three external displays: Up to two external displays with 6K resolution at 60Hz over Thunderbolt and one external display with up to 8K resolution at 60Hz or one external display with 4K resolution at 240Hz over HDMI

MacBook Pro 14- and 16-inch - Tech Specs - Apple

Are your ThunderBolt cables genuine ThunderBolt, with the ThunderBolt symbol at each end, AND shorter than 0.8 meter if Apple Brand cables, 0.5 meters if other brand cables?

If so, this article says it works.

MacBook Pro models with M1 Pro support two LG UltraFine 5K displays. MacBook Pro models with M1 Max support three LG UltraFine 5K displays.

Use the LG UltraFine 5K Display with your Mac or iPad - Apple Support

if your experience is different, contact Apple support or LG support or both.

The Mac uses a system that reminds me of “Plug and play” to determine what display is connected, and what its capabilities are.

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. "No signal detected" is generated by the DISPLAY, not by the Mac.

This query is only sent at certain times:

• at startup

• at wake from sleep — so momentarily sleeping and waking your Mac may work

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

• hold the Option key while you click on the (Detect Display) button that will appear in Displays preferences (from another display)

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

We can not escalate this, since we are users and these forums are not Apple folks responding for the most part.

I would suggest you reach out to Apple Support, and ask them to assist with an escalation of your issue, remember it may take several calls and patience to get to a higher level. My experience is that Apple wants to assist with fixing issues. Patience is a virtue when trying to sort out these type of issues.

there has been a pervasive issue when makers of "twin" displays have not provided unique serial numbers for each display, in the VESA-approved field. Historically, the issues were limited to the displays being randomly swapped for each other, because the Mac could not tell the difference.

Possibly in Self-defense, Apple has added an additional capability present in MacOS 11 Big Sur and later.

 menu >about this Mac > (system Report) > Graphics and Displays ...

... the detail for each display should now provide its serial number.

so check yours for UNIQUE serial numbers for each display.

If you have twin displays with the same serial number, or NO serial number, the manufacturer has work to do to fix that issue.

daniel157 wrote:

Apple Support said they have conflicting understanding because the tech specs (i.e. the apple.com tech specs) say up to 3 external monitors via USB-C Thunderbolt, but Apple support also told me that the MBP M2 Max only supports one (1x) external monitor.

The 13" M2 MacBook Pro only supports one external monitor. The 14"/16" M2 {Pro/Max} MacBook Pros support more than that.

Many people post in these forums about their attempts to connect two external monitors to the 13"/15" M1 & M2 MacBook Airs, and the 13" M1 & M2 MacBook Pros, even though the technical specifications for all of those Macs clearly state that they support only one external monitor. Maybe the Apple Support person became confused – and thought that you, too, were asking about a 13" M1 or M2 MBP, and gave the answer that was correct for it.

daniel157 wrote:

Apple INTEL display driver code was more advanced wrt to dual monitors because they DO work on Intel silicon MacBookPros but DON'T work on M2 Max...so is it really the serial numbers that are the problem?

I'm discovering this because I upgraded to M2 Max but it doesn't yet support anything but one external monitor at a time. It was the same situation with a Mac Studio I had for a minute but returned because of the same issue with supporting dual monitors. The M2 Studio could only do one external monitor at a time.

If the 14"/16" M2 {Pro/Max} MBPs actually only supported a single external monitor, and the M2 Mac Studios also actually supported only a single external monitor, I guarantee you that Apple would be flooded with complaints … and that there would be many articles about the problem on third-party sites that specialize in Mac news.

AFAIK, there is no such flood of complains, and there are no such articles. I am inclined to believe that there must be something that makes your configuration a bit unusual and different.

Given the symptoms (and trigger conditions) that you reported, Mr. Bennett-Alder's hypothesis that the problem is due to two displays reporting the same serial number (or no serial number at all) could be correct. Presumably the Apple Silicon Macs are using something like (Model ID + Serial Number) to form a unique identifier.

Upgrading to Sonoma 14.0 fixed the dual monitor support issue for the M2 Max.