iPhone X Face ID detection method: Open Face ID and entry (Settings - Face ID and Password - Set Face ID - Start using), use other Android phones or cameras with infrared filters to detect.
As shown in the following two figures:
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.
The range sensor and the proximity sensor & flood illuminator are attached on the earpiece cable, if there is no Face ID function after replacing the earpiece cable as a whole, even replacing the original one, the range sensor and the proximity sensor & flood illuminator on the original earpiece cable must be removed and replaced. Need to use the air pressure gun to disassemble, because the range sensor and the proximity sensor & flood illuminator are placed in a plastic housing, the risk of disassembly by the air pressure gun is high. It is recommended that skilled maintenance personnel operate on this.