Mac not detecting external display suddenly

Given the HDMI cable, Mac’s port, and the LG display all work fine independently with other setups, the issue seems to lie somewhere in how your Mac and the LG display are handshaking with each other specifically. The fact that Detect Displays does nothing and you've tested refresh rate/resolution options rules out common software misconfigurations. This could be due to EDID caching issues or a hiccup in how macOS is identifying the display.

I suggest you start by power cycling 'everything:'

• Shut down your Mac completely.
• Disconnect the HDMI cable and power cord from the LG display.
• Wait about 30 seconds, then reconnect the display's power.
• Boot your Mac and reconnect the HDMI.

If that doesn't resolve the issue, boot up your Mac in Safe mode to see if it can help:

• Use safe mode on your Mac - Apple Support

Create a new user account and log in. Sometimes user-specific preferences or corrupted display profiles can cause these issues.

f you have a USB-C to HDMI or direct USB-C to DisplayPort cable, see if the Mac sees the monitor through that. This can confirm if the macOS HDMI output stack is the culprit.

Finally, If none of the above resolves this issue, delete the EDID Cache:

• Go to `~/Library/Preferences/ByHost/` and look for files that include `com.apple.windowserver` and a long UUID. Trash those and reboot.

<< the resolution on my Mac primary display changes a bit. >>

that tells you you have Mirrored your displays together.

When displays are Mirrored, the Mac calculates ONE resolution that will be used to drive both displays in Exactly the same way. Occasionally, this compromise resolution is not quite good enough to support one or the other of the displays involved, and it will go blank.

if you can use either display, use settings > displays to un-Mirror displays.

if neither display is working try to use Command-BrigtnessDown to toggle the sense of "Mirror Displays".

The two inputs to the display are supposed to be the same HDMI 1.4 standard. if you move the cable to the other input on the display, does anything change?

Remember 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)

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"

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.

<< the resolution on my Mac primary display changes a bit. >>

Then try to use Command-BrigtnessDown to toggle the sense of "Mirror Displays".

exactly what make&model display is this?

do you get anything different when you type

Command-BrightnessDown

do you get anything different when you close the lid on the MacBook Pro?

cables 'shipped in the box' are notorious for being "lowest bidder" cables.

is it least a certified cable marked PREMIUM (a magic word that tells you it is good enough for this use) ?

if the other computer was running Windows, that Operating System has no minimum standard for good quality operation. If you think it's good enough, it is!

The Apple standard for its built-in hardware-accelerated displays, makes them suitable for full-motion video for production/display of cinema-quality video with NO dropped frames, and NO dropouts or partial-blank scan lines due to memory under-runs or other issues. When errors occur, the display may flicker or blank out entirely.

When the Mac queries the display and it answers, the display NAME appears in

Settings > displays ...

...regardless of whether you can get a coherent picture on the display.

if the name is showing, then hold the option key and see if the Detect displays button appears in the corner of the Display settings.

If no joy, there is something about the connection to the display that has gone wrong.

if the name does not appear, it is a more serious problem.

This is incredibly frustrating. Have tried all the suggested fixes and the issue persists. Certainly there must be a setting or something that was changed by the last operating system update. Can anyone shed some light on it? I also have the LG display earlier sold at the Apple store.