iPhone have so complex module to operate Face id function.These are the visible parts over LCD is that.
Ir camera & front camera but there is so many component is enter-relatively working for Face id.
1- IR Camera
2-Flood illuminatior + Proximity sensor in one flex
3-Ambient light sensor
4- Front Camera
5 - Dot Projector
How to check which section is failure that causes Face id function fail.We can diagonis by this way.Remember if the IR camera,front camera,dot projector all are encrypt by apple so,if the flex or FPC is damage can fixed but parts itself damage is not the solution.
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While light sensor on the phone is accidently damage put the phone brightness of the phone to final then you got more brightness then normal and when the brightness of the phone is in lowest level the use torch or flash light on the right side of the screen the brightness is automatically rises if it rises fails means light sensor is damaged which has also the most important in face id function.
The reason for the problem is that the FPC or flex accidentily damaged while iPhone screen is replaced.So,need extra care on while changing screen.
Causes-
Proximity and flood illuminatior flex or FPC - might be tore because this flex is on LCD side many technician or by changing LCD might be broken because apple uses special glue to fix that flex on LCD.