This guide explains how to update your iPhone 4 to iOS 4.3.1 safely without upgrading baseband. You will also read why it is important to preserve iPhone 4 baseband if you want to unlock your iPhone 4. Ultrasn0w is yet to be updated to support unlocking on iOS 4.3.1 but the untethered jailbreak has already been released.
In case you don’t know what your existing firmware version or baseband version is please first read How to Find iPhone Firmware and Baseband Version. Proceed only if you are on iOS version less than 4.3.1 and Baseband version 1.59.00.
This guide will work for both Windows as well as Mac users.
Why is it Important to Preserve iPhone 4 baseband?
If your iPhone 4 is not factory unlocked, you depend on software based unlock like Ultrasn0w. If you upgrade your iPhone 4 using stock firmware from Apple, then your baseband or modem version will be updated too.
You must note here that only iOS firmware can be downgraded. The Baseband of your iPhone cannot be downgraded. So, it is important that you carefully follow the instructions mentioned in this post to safely update iPhone 4 to iOS 4.3.1 and then perform jailbreak.
Does Ultrasn0w unlock iOS 4.3.1?
The answer is No. At the moment, only jailbreak tool has been released by the Dev-Team. If you follow these instructions you will be ready to perform the unlock as soon as Ultrasn0w is updated in Cydia, which can arrive any time this month.
Steps to Update iPhone 4 to iOS 4.3.1 Safely:
First download the latest version of TinyUmbrella using the links above and then install it.
Launch TinyUmbrella and connect your iPhone 4 to your computer using the dock connector to USB cable. If you are using this on windows and get an alert from Windows Firewall select Allow Access.
Now click on Save SHSH before we proceed. This will help us later in case we want to downgrade our iOS firmware version later.
Now we begin the Safe Upgrading process. Click on Start TSS Server.
If iTunes is running, TinyUmbrella will automatically force quit iTunes.
If you receive any error or TSS server does not start, then do not proceed. Click on Log and confirm that TSS server is running as show in following screenshot.
Now launch iTunes. Leave TinyUmbrella open.
Click on Alt+Restore on your Mac(Windows users press Shift+Restore) and point iTunes to the IPSW firmware file you downloaded previously.
iTunes will report Error 1013 after the restoration is over. Don’t panic. It means all is going as intended so far. Now we need to exit iPhone 4 from recovery mode.
Unlike earlier Fix Recovery button in TinyUmbrella will not work as TinyUmbrella hasn’t been updated to support this for iOS 4.3.1 yet.
So, download the standalone Fix Recovery application from here for Windows or Mac.
Extract the archive and launch fixrecovery43 by double clikcing on it. You will notice some lines of text on your iPhone 4 after which it should reboot with iOS 4.3.1 installed and baseband unchanged.
You can now proceed to Jailbreak your iPhone 4 using PwnageTool or RedSn0w. We will keep you updated about Ultrasn0w. Once updated you will need to download Utrasn0w from cydia and unlock your iPhone 4 using this guide.