Auto Brightness Not Working?

Mine works fine. Check the sensors on service mode (*#0*#) for hardware fault. Be sure sensor isn't covered /dirty...
I find them annoying most times as they dim too much, so rarely use that function.
 
It works if you go by what it says.


It says by YOUR manual adjustments. It then applies those adjustments in similar conditions.


So if you're always bumping up the brightness in dark conditions it's going to keep doing it as well. Thereby offsetting normal auto brightness.
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It never works - on any phone. All it does is make you think you're having a stroke or something is wrong with the power. I just adjust it manually.
 
It certainly works on my s8+, and as said, it memorizes your preferences. So keep bumping it up.

If you think it's broken, then shine a flashlight or other bright light directly on it, and watch the slider move by itself.