Judging by what we have learned during the teardown of Face ID, main circuits of the Face ID are exposed. Once water damaged or exposed to a relatively humid environment, there might be electricity leakage or short-circuit problem that can result in Face ID failure.
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And for a detailed repair process, Face ID parts are complicated and difficult to repair. A simple replacement can not fix the problem, not to mention that most Face ID parts are paired to the logic board. You need professional help.