Does your iPhone restart every three minutes, or is the time random every time it reboots? Maybe some parts of your phone is malfunctioning simply because it's too old. iPhones would checks if something is missing or malfunctioning three minutes after boot and if it finds missing or broken stuff it crashes and reboot to attempt to fix the issue. It also logs what causes it to crash in the form of crash logs. You can take a look in Settings - Privacy & Security - Analytics & Improvements - Analytic Data and find the crash logs, if you're running iOS 18. Previous generations of iOS should have a similar process, but I'm not entirely sure. Here's the guide for reading them and identifying the malfunctioning component:[br]
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[[iPhone Kernel Panics]][br]
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Hopefully yuo'll be able to find the issue and fix it. THere's simply no other way for an iPhone to safely backup data if it's boot looping. [br]
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If you didn't find useful information in the crash logs, then the problem might also be with the motherboard itself. Motherboards in phones pretty much always heat up, since there's no active cooling like fans (some gaming phones do, but they're expensive). If your phone is constantly under a heavy load, then the solder points might become loose and cause the phone to reboot every time they lose contact. It is a rare case among iPhones, but who knows?