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Repair and disassembly information for Samsung's flagship S21 Ultra Android smartphone, released in January of 2021.

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S21 Ultra bootlooping after screen replacement

Hey all, I'm new to the forum, but I'm dealing with a rather big issue.

I replaced the Display (and the battery since my kit came with one) on my S21 Ultra and everything worked fine for a couple of days until suddenly it started to bootloop. However, I get to the lock screen for a few seconds before it restarts which seems strange to me. Online, I was told to replace the daughterboard for some reason, which I then did and made sure to reseat everything. Phone worked fine for the next few days, and then it started bootlooping again. Any pointers on what the issue could be? Thank you!!!

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Replaced the battery and it was working fine for half the day and then started doing it again :(

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I'd remove the 'new' battery and refit the original and see how that goes. I've seen some garbage batteries supplied with replacement screens.

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I'll try and replace the battery ASAP. It's strange because it'll bootloop for hours until the battery finally dies. Could that really be the problem? Ribbon cable for it is fine and not loose, otherwise wouldn't turn on at all.

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@spiritedaway, yeah, it's entirely possible to get a battery with marginal electronics. I'm not an expert on Samsung phones, but it's pretty common on Apple phones for them to bootloop when they can't read a sensor on the battery and even occasionally on the screen.

So unless we get someone with more intimate knowledge of your phone to chime in, it kind of sounds like the replacement battery to me, but it's also possible the new screen could be contributing.

As @strongbow mentioned, if you still have the old battery it would be worth your while to put it in and see if the problem persists. If it does, then I'd try swapping out the display with the old one if possible, even if it's only partially functional.

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Christian will be eternally grateful.
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