Released on September 19, 2014, this 4.7" screen iPhone is the smaller version of the iPhone 6 Plus.

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Error 21, what's the first thing I should try?

So this phone shows error 21 during iTunes restore and says invalid serial number during 3utools restore. What should I try first? EEPROM reflow? Baseband Reball?

EDIT:

After reflowing the EEPROM, it shows 40 now.

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Yea I have a 5c that shows error 40 and my 6s went to restore when I tried to update

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@austinh2017 try what @refectio said below.

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This might help...

https://www.theiphonewiki.com/wiki/ITune...

From 3uTools

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

These errors typically occur when security software interferes with the restore and update process. Follow Troubleshooting 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.

I wouldn't get into reflowing IC's, I've suggested that to you before. Especially the Baseband...no good can come from that. I believe it's a software issue. Dig into the 3uTools documentation to find some options before playing with the hardware.

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The computer has a fresh install Windows 10, I don't have any security software and I have only programs I have downloaded before and trust. I think that something on the board is broken, or that the NAND is dead.

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I will try to Reball the NAND, I have everything to do it and will try my best. Do you know if I can use a 5s Reball stencil to Reball a 6 NAND?

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I believe so...just line it up and check. Have fun!

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Okay, after Reball, it is stuck in DFU mode. In 3u tools and iTunes it gets stuck on "enter recovery mode". So it won't go into restore, which I think is a NAND fault, since it doesn't have the required files, correct? I think I might have to get a new NAND chip or send it to someone to get it re-programmed. Didn't you say that you ordered a NAND programmer? How much would it be for me to send the chip to you?

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Yeah, I received my NAND programmer although I haven't played around with it much. Normally you can "repair" the NAND and you can read/write the serial number. Go to my profile and send me a private email.

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