Apple has paired a couple of assemblies to the logic board, presumably for security purposes to ensure that Face ID can't be hacked. One of these is the front camera, meaning only Apple can pair a new front camera with your existing logic board.
So the short answer is that there isn't any part you can buy to fix your Face ID if it isn't working; replacing any part of the Face ID system with one that isn't matched to the logic board disables Face ID.
The only thing an average person can do outside of taking the phone to Apple for repair is to replace all three parts together with ones salvaged from an original phone. Here's what's needed:
- Logic Board
- Front Facing Camera (Paired to that logic board)
- Proximity Sensor / Earphone (Paired to that logic board)
That being said, there are other repairs that can be made if Face ID stops working, but they generally require skill in microsoldering; not something just anyone can do. For example, the flex cable that connects the front camera to the logic board can get damaged and it is possible to replace it; it's just that it's very difficult without precise equipment and a particular set of skills.
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UPDATE as of 3/5/23
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I'd like to amend my previous statements as to the repairability of the TrueDepth camera system. As it turns out, the front facing camera, which is a part of the TrueDepth system, is not a part of the Face ID system. Since the camera itself is not paired to the logic board, it can be replaced and still have Face ID be functional. However, it is only sold by Apple as the front camera assembly, including the infrared camera and the dot matrix connector. Replacing it requires you to use brute force to open up the metal bracket holding the camera and replace the camera alone with a replacement part, leaving the other components intact.
Apple has paired a couple of assemblies to the logic board, presumably for security purposes to ensure that Face ID can't be hacked. One of these is the front camera, meaning only Apple can pair a new front camera with your existing logic board.
So the short answer is that there isn't any part you can buy to fix your Face ID if it isn't working; replacing any part of the Face ID system with one that isn't matched to the logic board disables Face ID.
The only thing an average person can do outside of taking the phone to Apple for repair is to replace all three parts together with ones salvaged from an original phone. Here's what's needed:
- Logic Board
- Front Facing Camera (Paired to that logic board)
- Proximity Sensor / Earphone (Paired to that logic board)
That being said, there are other repairs that can be made if Face ID stops working, but they generally require skill in microsoldering; not something just anyone can do. For example, the flex cable that connects the front camera to the logic board can get damaged and it is possible to replace it; it's just that it's very difficult without precise equipment and a particular set of skills.