I'm literally asking for a friend, who has a MacBook Pro 2016-2017 with no touch bar. (Big Sur 11.4, updated about 2 months ago).

They report that recently (noticed within the last week) when the laptop gets hot, usually when driving a very large BenQ monitor or perhaps when doing intensive numerical calculations, single pixel tall black lines appear across the bottom of the laptop's monitor in the Dock area. As the laptop continues to get hot over a period of 10 to 20 minutes, more isolated lines appear.

Their research shows that it appears to be a known problem with this model, and may be associated with something called a "T-CON" board.

Before considering a major repair, one bit of advice in this and this answer is to investigate the fan speed; is it going to full speed soon enough, does something need to be reset, etc. It also links to

Question: Is it possible to check the current speed of the fan on a 2016-2017 MacBook Pro? If that suggests it's not running fast when the computer is hot, is "resetting the SMC of your Mac" at least a safe thing to try?

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    First question to ask your friend is "How long since the machine was stripped & cleaned?" It doesn't matter how fast the fan is spinning if it's trying to fight a duvet's worth of dust bunny ;) After that, iStat Menus is the go-to fan/temperature app these days.
    – Tetsujin
    Jul 22, 2021 at 10:35
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    @Tetsujin update: good point; it's never been opened. So these are two very useful suggestions, thanks!
    – uhoh
    Jul 22, 2021 at 10:53
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    Running a Spindump (e.g. in Activity Monitor > Ellipsis in a circle in the toolbar > Spindump, or in Terminal) will show "Fan speed: xxxx rpm"
    – Redarm
    Jul 22, 2021 at 12:21
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    @Redarm for some systems (I'm on 10.15 myself) fan speed is not in the spindump it seems, but the command in this answer as well as one in the comments below it do show the fan speed directly.
    – uhoh
    Jul 22, 2021 at 12:37
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    @uhoh Good answers there. Also the suggestion to run a hardware test, or Apple Diagnostic from your first link is worth a go - fans are tested there too.
    – Redarm
    Jul 22, 2021 at 13:02

1 Answer 1


I’d recommend macs fan control (https://crystalidea.com/macs-fan-control) for seeing fan speed right in menu bar. You can also create a rule that cools more aggressively to prevent the problem and, as a byproduct, reduce thermal throttling.

As for resetting the SMC, it won’t harm anything. It’s one of the steps you do, along with Nvram reset, when an issue comes up that you can’t explain. It may not work but it won’t do any damage.

Edit: here is an image of the menubar display of macs fan control: enter image description here

This is customizable in the Menubar Display segment of the Macs Fan Control preferences

  • Can you add some more information about " macs fan control for seeing fan speed right in menu bar"? A screen shot or a link to some discussion? I've never seen such a thing. Thanks!
    – uhoh
    Jul 23, 2021 at 15:33
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    I added a section about the menu bar a month ago. Just realized that you might not get a notification for an edit.
    – anonymous
    Aug 22, 2021 at 16:11
  • Thanks for the ping; in this case I must have just missed it.
    – uhoh
    Aug 22, 2021 at 22:32

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