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Fix iTunes Error 21 While Downgrading iPhone 4S Firmware

When jailbreaking your iOS device, it is inevitable that you will run into some hiccups that puts the process into a complete stop. The usual errors that you will encounter are those that are returned by iTunes. As you may know already, there is a good deal of iTunes errors that you may encounter but one of the most common ones is the dreaded iTunes Error 21. This usually happens when you choose to restore your iPhone 4S using a custom firmware made by tools such as Sn0wbreeze or PwnageTool. In this guide, you will learn how to bypass such error.

Fix iTunes Error 21 in iPhone 4S

How to Fix iTunes Error 21 while Downgrading iPhone 4S Firmware?

First, in order to fix iTunes Error 21, you should know why it occurs. When faced with such an error, you should know that it means that you are trying to restore your iPhone 4S using a custom IPSW via Recovery Mode. The solution to this particular error is very simple, you just need to place your iPhone into DFU Mode.

What is DFU Mode?

DFU Mode VS. Recovery Mode
DFU or Device Firmware Upgrade mode enables an iPhone, iPad or iPod touch to be restored from just about any state. To be specific, the actual difference between recovery and DFU mode is that when you place your iDevice into DFU mode, it will not automatically load the iBoot. In contrast, when you place your iOS device in Recovery Mode, it will load iBoot which will make sure that the firmware that you are trying to restore to is equal or greater than the current one. So if you are trying to downgrade iPhone 4S firmware using an older one such iOS 5.0.1 from iOS 5.1.1, you basically can't do it with Recovery Mode. DFU Mode is the way to go.

How to Place Your Device into DFU Mode to Fix iTunes Error 21?

Placing your device in DFU Mode is very basic procedure. This is a very common process used when working with jailbreak tools such as Redsn0w or Sn0wbreeze. While it is indeed easy to perform, it can be quite tricky at times since it relies heavily on proper timing. For the benefit of those who don't know, just follow the steps below to place your device in DFU Mode:

  1. Connect your iDevice to a PC or Mac via USB cable and launch iTunes.
  2. Hold down the Home+Power button for 10 seconds.
  3. After 10 seconds, release the Power button but keep holding the Home button until iTunes detect your iPhone in recovery mode.
  4. Once your iDevice is in DFU mode, you should see absolutely nothing on display. It should be completely black.

iTunes Recovery Mode Detected
Even if you have successfully placed your iPhone 4S in DFU mode, iTunes will detect it as a recovery mode device and will return a message as shown in the screenshot above. Once the message has been displayed it will be your cue to finally proceed in restoring using custom IPSW and hope iTunes Error 21 doesn't show up. Hope you were able to fix iTunes Error 21 using this guide.

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