Auto brightness sensor does change the brightness of the phone but it will set it to full or at lowest, just 2 settings.
I mean after sunset which ever room or lightings u have at home it will stay at lowest and during day it stays at full whether u r outside or inside the house it doesn't make any difference.
Now a days during cloudy atmosphere if I am outside the house brightness goes lowest and as sun comes out of the cloud it goes full brightness.
So it does work for me but low or full brightness only.