1. (Figure 1) Can see the red light on the right side Lattice projector of the iPhone X, then the Lattice projector is normal. TheLattice projector is connected to the mainboard, which is not easy to be damaged when changing screens, most of the time it is damaged by water intrusion. Because the proximity sensor &flood illuminator attached to the screen, the earpiece is easy to get damaged when disassembling, most of the non-Face ID functions caused by such failures.
2. (Figure 2) You can see that the red light on the left side of the iPhone X is the range sensor. If it is not bright, the range sensor does not work or the earpiece cable is damaged, resulting in no Face ID function.
3. (Figure 2) You can see that the iPhone X left red dot proximity sensor & flood illuminator flashes is normally. If there is no flash, the proximity sensor &flood illuminator is damaged or the earpiece cable is damaged, resulting in no Face ID function.