MacBook Pro M4 Pro external LG monitor connection issue

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Level 1

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MacBook Pro M4 Pro external LG monitor connection issue

I have a new MacBook Pro M4 Pro that I received a few days ago. At my home office, I have an external monitor that is a 4K LG 32SQ780-WY. It is a "smart monitor" that doubles as a crappy TV. But as an external monitor it has been excellent. I had it hooked up to my MacBook Pro M1. When I would hook up the M1, the display just works as an extended display. When I hook up the new M4 via HDMI, the external monitor displays a message to add the monitor using mirroring. I did this, but the resolution on the monitor was terrible and the M4 recognized the monitor as a TV. I realized that the monitor was hooking up over my WiFi and not the HDMI. I checked the monitor for software/driver updates [it was up to date]. I then disconnected the monitor from WiFi, erased all history of previously connected external devices on the monitor, and powered it off and then on. I also checked my M4 was up to date and I powered it down and then on. When I reconnect via HDMI, the same message appears on the external monitor, but when I select the monitor as an extended display, it fails to connect and I get an error message indicating it is attempting to connect via Airplay but failing. So somehow the HDMI connection triggers the instruction on the external monitor, but the two devices cannot connect except with WiFi. I tried a USB-C connection, but the laptop and monitor would not connect, but the cable I had is not ACTIVE or Thunderbolt. I have ordered an ACTIVE Thunderbolt 4 cable, but I am still puzzled why the HDMI connection is not working. In the meantime, if I swap over to the older M1, the connection is fine.

I spent two days troubleshooting with Apple support and they were stumped. I contacted LG and worked with them but to no avail. They concluded that because the monitor works with other devices including the M1, it must be an issue with the Apple M4 Pro. Apple disagrees. And now I am stuck with a 4K monitor that I cannot use as an extended display.

Any ideas from the community are greatly appreciated. Seems a waste and expense to incur to need a new external monitor. I imagine there has to be a solution.

MacBook Pro 14″, macOS 26.0

Posted on Nov 1, 2025 11:29 AM

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Answer 1

Top-ranking reply

Grant Bennet-Alder

Level 10

139,643 points

Nov 1, 2025 11:39 AM in response to jcmaerz

The Mac does not rely on Windows-like side-loaded "Drivers" which are actually packages of resolutions and settings for a specific display. Instead, it goes straight to the immutable source -- it asks the display itself.

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.

NB >>

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.

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Answer 2

Nov 1, 2025 11:38 AM in response to jcmaerz

that LG 32SQ780-WY appears to be a $K UHD display 3840 by 2160 cable of 10 Bits/color HDR

connectivity includes:

2x HDMI 2.0

USB with up to 65 Watts power delivery

It is NOT a Thunderbolt Display, and will not gain any increased capabilities when connected with a ThunderBolt cable of any description.

HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"PREMIUM High Speed HDMI cable" or that + "with Ethernet" (up to 4K at 30Hz) --OR--

“ULTRA High Speed HDMI cable" or that + "48G" (supports higher resolutions and backward-compatible)

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

HDMI was invented for HD TV sets. it works great at its original resolution of 720i or 720p. At higher resolutions, it quickly develops issues that are complex to solve, and the cables and adapters required to solve are NOT intuitive.

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Answer 3

Nov 1, 2025 4:55 PM in response to jcmaerz

LG – 32 Inch 4K UHD Smart Monitor with webOS and Ergo Stand (32SQ780S-W)

3840x2160 pixel resolution, 60 Hz refresh rate, 10-bit-per-channel color depth (1.07B colors).

Video inputs: One USB-C (DisplayPort) input, two HDMI v2.0 inputs.

You should not need a Thunderbolt cable to connect this monitor directly to a Mac – just a good-quality USB-C cable capable of carrying 4K video. A Thunderbolt 3, 4, or 5 cable should be OK, it's just that you will be paying extra money for the higher construction quality needed to support Thunderbolt data rates.

HDMI v2.0 has bandwidth limitations such that you cannot have all of

4K resolution
60 Hz refresh frequency
10-bit-per-channel color
RGB 4:4:4 encoding

at the same time. If you compromise on RGB 4:4:4 encoding, that makes some adjacent pixels be forced to the same color, effectively reducing resolution.

The typical tradeoff is (60 Hz, 8 bits per channel) vs. (30 Hz, 10 bits per channel), and there, I believe that most people would be happier with the 60 Hz one.

That said, I would think that a M4 MacBook Pro would have no problem making either a USB-C connection or a HDMI connection to this monitor. You would want to be sure to use a cable rated for 4K video – preferably one which was 1 meter (~ 3 feet) long, or less. Some HDMI cables are only meant for 1080p at best.

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Answer 4

Nov 1, 2025 5:03 PM in response to jcmaerz

jcmaerz wrote:

When I hook up the new M4 via HDMI, the external monitor displays a message to add the monitor using mirroring. I did this, but the resolution on the monitor was terrible and the M4 recognized the monitor as a TV. I realized that the monitor was hooking up over my WiFi and not the HDMI.

That LG set supports AirPlay 2. AirPlay 2 is not nearly as good as a hardware-supported cabled connection, but if the TV was set up to allow AirPlay connections, the Mac would have seen it as an AirPlay device.

It's less clear why you got a message to use mirroring instead of HDMI. Was that generated by the Mac, or by the TV?

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Answer 5

jcmaerz Author

Level 1

4 points

Nov 1, 2025 2:28 PM in response to jcmaerz

I very much appreciate your effort and suggestions. I'm still not having any luck with USB-C or even the HDMI. I can't figure out why it works fine with the MacBook Pro M1 but not the new MacBook Pro M4. Is it your guess that maybe the M4 is not working with this monitor via HDMI? Apple thought it should work no different, but it will not.

Apple told me to make sure the USB-C cable was Thunderbolt and they had the specs on the monitor, but that does not appear to be good advice.

I used the monitor settings and monitor confirms that the monitor is set for HDMI/USB-C connection. That is the only option I could find. I note that the monitor displays a message that says "HDMI PC2" when I have the HDMI connected. I can't get the USB-C to connection to be recognized by either device. USB-C connection works fine with M4 on my external monitors at office, so the issue is clearly an interaction between the M4 and this monitor.

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