MacBook Pro only connecting to one monitor despite being hooked up to two

Awhite130 wrote:

I have a 14" MacBook Pro, Nov 2023, on an Apple M3 Pro chip. The tech specs say that it is compatible with dual monitors. I've used a Sonnet Thunderbolt Dual Display Port adapter to connect my machine to two new Dell P2422H 24" LED monitors. When connected, only one monitor can be found on my mac in the "displays" tab and only that one works. The other one appears unconnected despite being on and plugged into the adapter. Any ideas?

SonnetTech has two Dual DisplayPort adapters with very similar names.

The Thunderbolt Dual DisplayPort Adapter depends on the built-in hardware support in your Mac. It should work fine with a 14" MacBook Pro that has a M3 Pro chip (which you say you have) or a M3 Max chip. It would not give you the ability to attach two displays to a 14" MacBook Pro with a base M3 chip.

The DisplayLink Dual DisplayPort Adapter for M1 and M2 Macs uses the DisplayLink "workaround" technology to enable connection of two external monitors even to Macs whose hardware does not support two. If you're using this gadget, at least one (and probably both) of the video outputs are based on the second-class workaround, so will not work unless you have installed the corresponding DisplayLink software on your Mac.

https://www.sonnettech.com/product/accessories/adapters.html

The Dell P2422H s a 24", 1920x1080 @ 60 Hz monitor with DisplayPort, HDMI, and VGA inputs. It comes with a 6 foot DisplayPort to DisplayPort cable.

Troubleshooting

First, I would suggest double-checking that your 14" MacBook Pro has the M3 Pro chip and that your SonnetTech adapter is the Thunderbolt one.

Second, in other in threads, I've seen Mr. Bennet-Alder recommend using short video cables to connect monitors to Macs. Apparently modern Macs want to see error-free connections to displays. If they see that there are errors on the connection, they will often reduce the resolution of the connection or drop the connection (planking the display) in circumstances where older Macs ("versions of macOS") and where Windows PCs would just soldier on, ignoring the errors.

Using low-quality video cables, or long video cables, can increase the chances of errors. I'm not sure that 6 feet is bad, especially considering the relatively low resolution (1920x1080) of these monitors. Lots of USB-C to whatever (DisplayPort, Mini DisplayPort, HDMI, single-link DVI, VGA) adapter cables are six feet long. But I can''t completely rule out the possibility that a shorter cable (say, 2 or 3 feet long) from the adapter to the display might help.

Awhite130 wrote:

I do not have a cable with the Thunderbolt adapter on one end and another end that will connect to the monitor. That is why I purchased the Sonnet. The cables that came with monitors won't connect to my macbook pro. That is the only thing I have to connect them, so I'm not sure what would happen if I tried to connect without the adapter.

The cables that came with each of your monitors are

• DisplayPort to DisplayPort (for video)
• USB to USB (probably USB 3.0 B (monitor) to USB-A (computer)) (for connecting downstream USB hub ports)

These cables won't plug directly into your computer, and you should not try to force a "connection".

I believe what WheelieNick was suggesting was

• USB-C to DisplayPort adapter cables – to bring out the native DisplayPort signal, or
• USB-C to HDMI adapter cables – to bring out the native DisplayPort signal and convert it to HDMI

Whether you want to try those first, or try shorter DisplayPort cables with the SonnetTech dual adapter that you're using now, is up to you. Either way, it may take some experimentation to figure out what's going on.

Aside: If you decide to make USB hub connections to either of the monitors, you could use various forms of USB-A to USB-C adapters; or you could substitute a USB 3.0 B (full-sized B) to USB-C cable for the cable in the box.

There is another possibility here that I ought to mention. It's a corner case – may not apply to you – but it can be non-obvious that it might be the source of a problem.

You say that you have two Dell P2422H monitors.

Manufacturers are supposed to program each monitor with a unique serial number in the EDID ("extended display ID") that is available for the computer to query. This helps the computer to keep track of which monitor is which.

Sometimes manufacturers will take short cuts and they will load flash chips for a whole bunch of monitors with a single firmware image containing a single, NON-unique ID. This happens most often in the context of producing runs of (you guessed it) the same model of monitor. Then when some customer buys two monitors of that type, there's a chance that they will get two monitors with the SAME ID. Oops! That might cause unwanted behavior, depending on how much the computer relies on the broken "promise" that each monitor will have a unique ID.

What is happening when you're trying to connect it without the dual display port adapter? So just 2 monitors via separate USB C cables?

how long are the DisplayPort cables from adapter to each display?

what displays at what resolutions"

Grant Bennet-Alder wrote:

how long are the DisplayPort cables from adapter to each display?

If they are the ones included in the Dell P2422H monitor boxes, they are 6 feet long (according to Dell's product description page) and are DisplayPort 1.2 cables (according to Dell's online user manual).

what displays at what resolutions"

The OP said they were Dell P2422H monitors – so 1920x1080 pixels @ 60 Hz.

Still only connects to one external monitor