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Sometimes, main display only work when connected to external monitor

Sorry for my English so bear with me.

The main display suddenly disappear until I connect it to an external monitor, and then both main and external monitor will work again. The computer still works while the main display loses it image, the monitor still has it backlight, the apple logo is still glowing, I can still adjust it brightness but no image, and I can still play music. Only if I connect it to external display the main screen suddenly works again, so both main and external display works again. And if I unplugged the external display, the main monitor will lose its image again, but it does not happen instantly, it will take probably 4 to 5 sec before it went black again. I dunno know, maybe there is a bug in graphic switching?

This issue only happens 2 times during my 5 year owning this macbook. The only way to resolved this problem is to restart the computer, and it will work fine again. I couldn’t remember the first time it happened, maybe a month ago or so. This does not happen during startup, this only happens maybe after using the computer for hours or even days without shutting it down. For now it’s not a huge issue for me because this happens so rarely and restarting the computer fixes the issue. But it still bothers for me because macbook pros are pretty expensive and I’m not sure if this issue will get worse in future or maybe it’s a symptom for much bigger and expensive repairs in future.

I’m not sure if this is a hardware or software problem, or a virus I guess? Or maybe a bug on graphic switching?

Device: Macbook pro mid 2015 with m370x
OS: MacOS Monterey latest

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Your issue is within the MUX circuitry. Basically, the video signal needs to be routed to quite a few ports. Between the internal display you have two Thunderbolt ports which could be supporting either a Thunderbolt or DisplayPort display and then we have a HDMI port. You could have one or more in mirror mode or have them in extended screen mode. Thats a lot of directions and options!

One other variable is the lids Hall sensor which can also effect things. It can be defective telling the logic the lid is closed so the video signal is not sent to the internal display. In this case since the backlight is still visible that’s not the case.

What you are seeing is a bad termination condition. So when you plug in the external display its adding load so the signal is able to work for both displays, then when you disconnect it takes a bit of time for the loss of load to revert to the internal display.

Your logic board needs repair but someone with the needed knowledge to diagnose where in the circuit the issue is located. Your other option is just replacing the logic board.

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I finally replicate the problem, it happened again this morning. So whats happening is as soon as it happens, going back from dGPU to iGPU the problem will occur. The reason why the main display works again when plugging to external monitor is because it forces the macbook to use the dGPU, so unplugging it will trigger again the problem because of iGPU. Now, I no longer need to restart my macbook, I just have to turn off the “automatic graphic switching“ and use dGPU at all time.

Is this a hardware or software problem?

von

@cleancoder - sadly this is a hardware level issue.

von

In your experience, will this get worsen over time to the point that my computer will no longer be usable? If so, how soon?

von

@cleancoder - If you can live with it, it should stay this way.

von

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CleanCoder wird auf ewig dankbar sein.
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