I replaced screen on my iPhone 5s Touch ID failed luckily had the flex of original fingerprint scanner put that back on the phone Viola was working again, can definitely say it’s chip pairing with logic board. Got a iPhone 6 Plus to try on next will let you know the results soon
It's the U2 ic on logic board which has failed as stated by jessabethany needs replacing unless your good at reflowing/ replacing ic's i wouldn't bother it's a real b**lache. I would just try and get a working second hand logic board unless you have some real important files on phone then worth getting repaired hope it helps thanks
Firstly I would take the motherboard out and give it a good clean with alcohol then check the actual battery connection on the board if its loose or residue. Try new battery after. Error 21 is battery related hope it helps
Take battery out keep phone plugged in charger test battery terminal on board with mutimeter if you get a reading then your board is ok battery is faulty if you don't get a reading most likely 1 of ic's have gone on board most likely need reflowing or replacing
I have same problem with my iPhone.i think its problem with power management ic. If the phone has not been opened send it back to apple with original things and they should sort it out. Update: I got a new battery charged it my mates phone and put charged battery in my iPhone it worked as long as I didn't let battery die would charge up aswell but once battery died it just would not boot. I just bought 2 new batteries but its just not booting up nothing just dead.am hoping to reflow power management ic.but I'll try doing what old turkey said first though let you know outcome. But if you have original things in it send it back to apple doesn't matter if it is out of warranty.there has been lot of problems on ios 7 with battery so they should sort it for free worth a shot
If your phone was water damaged you have to change battery hopefully thats your problem. Error 21 is always to do with battery or battery connections. Replace battery if that doesn't work check battery connector on logic board might be damaged hopefully should sort it out had the same problem.
Try cleaning with isopropyl alcohol 90plus %. If you are comfortable with opening the phone. Give it a good clean with toothbrush or similar should come back to life hopefully. But be careful in disconnecting battery from connector it's very fragile