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iphone 6s touch id problem
Have an interesting case of iphone 6s not being able to read the touch serial id. The touch id comes up blank in 3uTools verification report, however the same screen/button touch id serial can be read fine (mismatched, of course) when connected to another known-good phone. Thus it seems to be the logic board problem.
Mesa connector diode mode values on logic board look reasonable. Filters seem intact. PP1v8_Mesa and PP3v0_Mesa voltages are present on (disconnected) connector. I proactively replaced U4040 and checked nearby connected components and its B1 pad resistance to VCC_MAIN. All seems normal. Yet, the button touch id is failing to read its serial number on phone start.
Could you suggest troubleshooting ideas? I did reset the phone in recovery mode -- could it have disabled the touch id in firmware similar to how baseband would get disabled in firmware when there's a physical issue?