when you opened the phone and removed the LCD screen assembly to pull the old battery, did you disconnect the battery first?
when you put the phone back together, reconnecting the motherboard to the new LCD screen, did you connect the battery last?
if you answered “no” to either or both, it’s possible that you may have shorted something out working the connectors with the battery attached. that’s just a possibility.
another situation could be some dust or debris that got into the FCP socket during the repair and is blocking a contact or two. i’d disassemble the phone again. visually inspect all the connectors (plugs and sockets) that you disconnected/reconnected. personally, i’d use a touch of 99% IPA and a soft toothbrush to clean the contacts. if you’re not comfortable doing that, you don’t have to. or, even though apple says not to use compressed air, you can very, very gently blow a low pressure puff (a very light press) across the connectors to blow any dust out. do not use high pressure and do not use your own breath (you’d be surprised how much micro-droplets of saliva actually come out in what you think is a dry breath.
i’d actually put the original button back in and re-test, but bear in mind the home button connector as well as all the flex cables are very easy to damage if you’re not careful. hopefully this isn’t the case, but it’s a possibility
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