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Question: Can "unknown error 21" be fixed ?? (iphone 4)

Recently my iPhone 4 just reset itself out of nowhere and wouldn't turn back on. I placed it on a dock & the connect to itunes screen showed up. I plugged it into my computer and when it began to restore it stopped and the unknown error 21 window popped up. This has occured on 3 different computers & now I'm unsure how to handle this.

-Please help.

iPhone 4, iOS 6

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Jun 20, 2013 1:59 PM in response to _Alama_ In response to _Alama_


Error 20, 21, 23, 26, 28, 29, 34, 36, 37, 40

These errors typically occur when security software interferes with the restore and update process. FollowTroubleshooting security software issues to resolve this issue. In rare cases, these errors may be a hardware issue. If the errors persist on another computer, the device may need service.

Also, check your hosts file to verify that it's not blocking iTunes from communicating with the update server. See iTunes: Advanced iTunes Store troubleshooting—follow steps under the heading Blocked by configuration (Mac OS X / Windows) > Rebuild network information > Mac OS X > The hosts file may also be blocking the iTunes Store. If you have software used to perform unauthorized modifications to the iOS device, uninstall this software prior to editing the hosts file to prevent that software from automatically modifying the hosts file again on restart.

Jun 20, 2013 1:59 PM

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Mar 17, 2014 1:32 PM in response to _Alama_ In response to _Alama_

Typically Yes That k-base document will resolve, OR at least help....

This is how you see if it is your computer software causing the issue OR if it is the battery in your unit.

attempt to restore the device, when you recieve error 21 (unable to restore "device" unknown error occured 21)

make sure your device has been plugged in long enough to recieve some sort of charge. ( computer, dock, charger makes no difference)

Unplug the unit.....

Did it turn itself off and act like a brick? If Yes, then your battery itself is faulty and needs replaced!

iphone 4 owners may purchase a replacement from amazon for around 5 dollars and do the replacement them selfs ( walkthrough on youtube) "how to replace battery on iphone 4"

other devices, same behavior, depending on your modle of IOS device you have may dictate if you can repair this or IF you need to take to apple service provider / or call apple care 1-800-APL-CARE.

If your device is less than a year old OR if you have an apple care protection plan ( AND the unit has not became wet) you may send it in to apple for a battery replacement. ( if it had been damaged depending on the damage ther may be a fee. If it is outside of warrenty generally 79$ {iphone} is the pricing you are looking at if the unit is outside of coverage.)

Hope this helps!!! I was able to solve my error 21 by simply swapping out the battery THEN restoring the unit via itunes without further errors!


Mar 17, 2014 1:32 PM

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Mar 17, 2014 1:37 PM in response to OnceUponAnAppleCare In response to OnceUponAnAppleCare

OnceUponAnAppleCare wrote:

I was able to solve my error 21 by simply swapping out the battery THEN restoring the unit via itunes without further errors!


Great, so you fixed your problem by forfeiting any your ability to ever have it serviced by Apple again... Wonderful for you. NOT good advice for people who come here looking for help.

Mar 17, 2014 1:37 PM

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Mar 17, 2014 1:46 PM in response to KiltedTim In response to KiltedTim

again, If your device is covered by any warranty or applecare, then the battery replacement IS free. If it is outside of its warranty, you can pay them for replacement. in no way will this forfeit the ability for apple to support the unit you own.

alturnitivly - you can attempt to restore the device on a different computer using a different network. OR turn off firewall, and delete the IPSW file causing itunes to redownload the restore file again.

only if you are sure your battery is dead - connect to power ( on? yes? ) disconnect ( off yes?) will not turn back on? no? the battery is gone and will need replcaed regardless the route you chooose, as DIY or apple.

Mar 17, 2014 1:46 PM

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Mar 17, 2014 1:53 PM in response to OnceUponAnAppleCare In response to OnceUponAnAppleCare

and NO it absoultly does not FORFEIT the ability for apple to service the unit by sending it to apple care! and if the unit is BER ( beyond economical repair) or have gotten wet then apple will not service it at that point and it will be paid repair. Suppose you are internal apple - please review your eligibility scope of support it was once cp28

(establish eligibility for service)

In most cases repairing a unit outside of warranty will be cheaper than paying apple to replace the device.

I have spoken to countless persons seeking assistance my time at applecare. I would never mislead or take away from their wonderfully amazing support team.

besides, if the unit stays on after disconnecting then its not the battery and other options to bring the unit back to life are available.

this is not the fix for 100% of error 21, but it HAS been the fix in my experience several times.


Mar 17, 2014 1:53 PM

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Mar 17, 2014 2:15 PM in response to KiltedTim In response to KiltedTim

AGAIN, the warrenty is a legal document expressing a MFGs obligations to a product.

YES if i open a "closed" unit and am not apple certified ( which i am and owner of a local service provider/repair center which is besides the point and the reason i left it out) then apple WILL IN NO WAY FURTHER SUPPORT THE DEVICE IN ANY WAY in this you are correct.

HOWEVER, if your device is OUTSIDE the warrenty or its support you MAY repain the unit yourself.

79$ iphone battery plus 29.95$ shipping ... apple replaces battery (done)

8 dollars ( includes shipping) remove a total of 3 screws place in battery, place screws back ( done)

still this information is credible as it is proven ( dont takt my word research it google "iphone4 error 21 battery")

this may only speed up the process by calling applecare for a bettery replacement.

info is info .. thats all this is. Sorry if i imposed on your turf.

Mar 17, 2014 2:15 PM

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Mar 17, 2014 5:11 PM in response to OnceUponAnAppleCare In response to OnceUponAnAppleCare

The warranty is void and any rights to have it serviced by Apple either in or out of warranty are forfeit if the phone is opened by an unauthorized party, including yourself.

You can legally open it up and do whatever you want, however, the moment you do so, any warranty or Apple care agreement is void and Apple will no longer service it AT ALL.

Mar 17, 2014 5:11 PM

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Feb 4, 2015 5:21 PM in response to _Alama_ In response to _Alama_

ok I have the same thing happen to me I at this very just fixed this problem as I type too you. This what I did first Get you a New battery if you haven't all ready plug the new battery in to your phone let it charge for sometime ( 2 or 4 hours at least) ok some people say that alone fixed there's but not mine. Ok you should be seeing that Itunes logo at this time,now to check and see if your phone has power since it has been charging for some time, now you should be able to unplug the charger and your phone stays on, you should still see itune logo when you unplug it if not, then do this and this is what i did that know one else did. ok so open your phone up like your going to take the battery out make sure you have the phone charger hooked up to the phone and usb in too the laptop now unhook the battery from the phone wait a few secs.then plug the battery back to the phone,now unhook the battery one more time but this time the phone should stay on, if your phone stay's on then you are on the right track.ok last thing to do, hook the battery up to the phone. now all you should have to do is go to itunes and run the restore and up date. the reason you keep getting the error code 21 or 2001 is because after the restore bar fills up and then the phone screen goes black,and then it cuts off and the restore has a error,the phone screen should go black but NOT CUT OFF thus that's what the code error 21 and the 2001 both are the same thing. I hope this helps, what it come down too this is a battery problem get a new a battery let it charge even if you dont see the charging logo it is charging... hit me back if anyone is too

Feb 4, 2015 5:21 PM

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Question: Can "unknown error 21" be fixed ?? (iphone 4)