My iphone 4s wont turn on with out being pluged in and when i unplug it it dies instantly i pluged it in put it in dfu mode and tried to restore it hoping it might help but its stuck in dfu and has restore error 21 help!!!
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After reviewing your post, it sounds like you had one issue and then got a second trying to resolve the first. A frustrating situation for sure. The first article linked has what to do if it is not powering on unless plugged in. The second article addresses the error you are receiving when trying to restore.
iPhone: Hardware troubleshooting
Will not turn on, will not turn on unless connected to power, or unexpected power off
Resolve iOS update and restore errors
Check for hardware issues
Related errors: 1, 3, 10, 11, 12, 13, 14, 16, 20, 21, 23, 26, 27, 28, 29, 34, 35, 36, 37, 40, 1000, 1002, 1004, 1011, 1012, 1014, 1667, or 1669.
Have a nice day,