How do I reconnect HDMI second display on MacBook Pro?

fcmattar wrote:

I could fix using this suggestion by @sangy987

macOS Sequoia 15.3 wont let me detect my … - Apple Community

to summarize that:

Finder > Go menu > Go to folder; Enter

~/Library/Preferences/ByHost and press return

scroll WAY down until you see the file named:

com.apple.windowserver.< hex string >.plist

drag that file to the Trash

Restart

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.

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)

For your experiments, be certain that you are aware of those requirements.

It sounds like your internal display and external display are set to "Mirror displays". When this is true, the Mac computes a compromise resolution that it thinks BOTH displays can achieve, and use it for BOTH. Then both displays are sent the same data, in lock-step.

Sometimes one or both displays can not quite achieve the computed resolution for some reason. Then one display might drop out.

The solution is to turn OFF "Mirror displays" in

Settings > Displays

if you have no working displays, Command-BrightnessDown is said to toggle the sense of 'Mirror Displays'.

What model-year MacBook Pro?

What make&model display?

What cables, adapters and docks are you suing to make the connection? please mention EVERY item in the chain.