Macbook Pro 14 2021 - A2442 - 820-02098-A , Internal display not turning on

Dear collegues,
the owner of this machine brought it to me saying that suddenly one day the display didn't turn on anymore, he tought that all the machine was damaged but it turned out that the MacOS starts good and stays on, and that connecting it to an external display it works without problems.
If you take a look into general settings, you can also see that the internal retina display is recognized as connected by the main board but for an unknown reason it doesn't turn it on.
I've tried to put a bright light towards the screen but I can't see anything, any shadows or apple signs during startup, so it seems that the matrix is not even driven.
I've found schematics and boardview of this board (820-02098-A) but it's difficult to diagnose and also to take measures because the main video signals are in the internal part close to the keyboard :/
The board is completely clean, no traces of liquids, no oxydizations anywhere, no burned components, the external chassis is clean and has no signs of hits or scratches, and the owner said that it wasn't exposed to heat or cold or water, but always used in his studio.
Sandly I have no other similar displays to try on, nor other similar mainboards, so I have to find the fault on this machine itself.
I've read that the LID sensor is something special on these machines and it has communication with CPU which needs UR330 outputs to control TP/KB, internal video and digital microphone, and if you change it you have to program and calibrate it :o
I've tried detaching the lid sensor but the behaviour is the same (I know that it can't prove anything because maybe I need a completely working sensor programmed for this mainboard to have a proof).
Some of you have had similar problems on a similar machine or can point me to a direction?

Thank you for your time

If disconnecting the sensor did not help, try revive in DFU. If this won't do the trick then the display is likely faulty.

Thank you master Vogelek23, I will tri it.
I'm not so expert about these "new" Apple machines, can I ask if this DFU revive can be dangerous if not finalizes good?
Could it lead to a worst situation?
I ask this because right now the MacOS starts and works without any problem with an external screen, I don't want to get things worst :(
A secondary strange thing is that whenever I choose "Shut down" from the desktop o from the initial startup screen, the system shutsdown, the orange light on th chargin tip fades out and then after 2 seconds it comes up again and MacOS startsup automatically :/
Should it shutdown and stay like that until the power button is pressed again?

Mobotool hat geschrieben:can I ask if this DFU revive can be dangerous if not finalizes good? Could it lead to a worst situation?

Revive does nothing to the OS. It only re-installs the core of firmware. However, nobody can guarantee that nothing will happen. It is extremely unlikely to get the things worse, but still possible.

Mobotool hat geschrieben:I don't want to get things worst

Not possible with Apple devices. Sometimes you need to fully restore the firmware to fix the problem, and this means data are gone. That is why the customer is expected to create a regular backups of their data.

Mobotool hat geschrieben:A secondary strange thing is...

Not too strange on Apple devices. Whenever you choose to shut down the computer, after switching off it checks if the lid is still open - if so, it starts up automatically because it "thinks" that you require the computer up and running. If you choose to shut down the computer and close the lid, it does not turn on until the lid has been opened. Just a "normal" feature.

Hi, I've back up the ssd and made firstly a Revive then a Restore of all the system , the operations completed successfully without errors but still the display doesn't come on...
Connecting to an external hdmi monitor, graphic is clear, no problems or glitches, and still in general settings I can see both monitors connected to the system, the external tv and the internal Liquid Retina XDR.
When I close the lid, in general settings internal display fades out and remain only the external tv.
Strangely, I'v tried disconnecting the three flat cables that goes to the internal display (JP600, JP400 and JP900) and , from external monitor, MacOS still does see the internal display as connected
Can someone shade a light about this or what to do further?
Can I make some measurements or tests on the internal display or the mainboard before buuying a new one (I know that I will loose True Tone with the new one...)?
Thanks

At this stage of diagnostics I believe that the display is faulty, unfortunately.

Hi Vogelek,
thank you for your quick answer.
Can I ask if you have an explaination of why even if the display is disconnected from the mainboard, MacOS stll does see it as connected?

I can only guess that the LCD panel is no longer considered as a separate I/O device, but is treated as the end part of an LPDP interface. Unfortunately, there is no possibility to get into the CPU's structure and see what kind of modules does it contain. What is the main problem here is that there is no single test point available to see the state of LPDP_INT_HPD signal when the panel is plugged in. When unplugged, the signal should stay low (0V) and when plugged in, the signal should rise to high (likely 1,8V). If this differs, it may also be a CPU fault. Easier is to replace the LCD panel than the CPU.