Surface pro 3 Brightness control missing

Previously, I've not used brightness slider on the right panel, but after windows 10 automatic updates adaptive brightness control stopped working and I had to use the slider on the right panel. So I cannot understand why I've paid for that light sensor of adaptive brightness control to be disabled. Microsoft isn't that of the previous quality unfortunately. No solutions on the internet and on this portal helped with the problem.

Hmm yes it's still perfectly possible the brightness will fail again for me, but it hasn't yet after a few days :)

Did you try the Surface Diagnostic Tool? Though it didn't fix it itself (in fact it set the brightness to 0%! I suppose it's a solution of sorts if you can use the Surface only in dimly lit locations). The Reset after fixed it.

I realise this is part-black-magic - but it might be worth trying.

This issue makes the Surface into a redundant gadget, if I have to be plugged in I might as well stick with my lovely big 17 inch laptop. It's not as though Microsoft can blame anyone else - they 100% make the hardware and the software.

the brightness slider can not be moved and is shown dimmed as if it is inactive.

I have the same problem. I also tried disconnecting the type cover, reboot and change. But I still can't change then. I have the Surface Pro 3 i7 8gb

I also tried the following methods:

https://www.tenforums.com/tutorials/70157-enable-disable-adaptive-brightness-windows-10-a.html

Option 1: those options aren't available

Option 2: Doesn't work

Option 3: I did this one, but didn't change anything

Option 4: I can't do this because I because there is no Intel Graphics Settings, I tried to install the Intel drivers manually but I can't install them (https://downloadcenter.intel.com/download/28662/Intel-Graphics-Driver-for-Windows-15-40-?product=75114)

https://surfacetip.com/how-to-unlock-power-plans-on-surface-devices/

I changed CsEnabled to 0 in regedit, but that also didn't help

I now do have more power options and I can change them (but there is still no adaptive brightness option). But I can't choose any of the power plans (it gave an error)

https://windowsreport.com/adaptive-brightness-windows-10-wont-turn-off/I did the 5. Disable Adaptive brightness in the Registry but that also didn't help

This is really annoying. I don't want to use adaptive brightness, manual is fine with me. I just want to be able to adjust the brightness. Even on battery saver the brightness looks like it is at full.

My SP3 got the exactly same issues after updating to 1909. I've tried most of the solutions mentioned in this thread, and none of them worked. Resetting PC with keeping personal files was unsuccessful for unknown reason. 

Besides, I tried the "Surface Brightness Control" tool (https://docs.microsoft.com/en-us/surface/microsoft-surface-brightness-control), which is designed to "lower the overall carbon footprint" by auto dimming after some predefined inactivity period. Sometimes it can dim my screen to 20%, but most of the time it's not functioning. (I had to create the registry keys by myself, but it's still not working as expected after tuning the values.)

Finally, as suggested by this post: https://www.surfacetablethelp.com/2019/12/brightness-slider-control-missing-in-windows-10-on-surface-pro-3.html, I tried the third-party tool PangoBright. Win 10 is not mentioned on its induction page, but surprisingly, it works. It can adjust the brightness in 10% interval from 20% to 100%, which is good enough for me. (Seems it conflicts with "Surface Brightness Control", so that tool should be uninstalled.)

However, PangoBright is not perfect. The brightness still goes to 100% when showing the start screen and the side bars. And other problems (empty Power and Sleep setting, power button doesn't turn off screen) still exist. For now I have to live with this until Microsoft fix this stupid bug.

Update:

After using it for a while, I realize PangoBright doesn't change the screen brightness actually, but just change the color for all applications. So it cannot save any power consumption by LCD. 

In my last attempt, I managed to reinstall Win10 using the image downloaded here, which still allows me to keep my files. I don't know what happened, but after reinstallation everything looks normal: the version is 1909 and auto updating seems don't break anything.