NOTE: This guide is for those of you who are on a jailbroken iPhone 3GS firmware 3.0 or 3.0.1. If you have updated to iPhone 3.1 from iTunes but have saved your SHSH file to Cydia then you can still jailbreak but you’ll first have to downgrade your firmware back to 3.0 or 3.0.1. In other cases, you are locked out.
First thing we need is Custom Firmware: Google for iPhone 3GS Custom 3.1 download or “iPhone2,1_3.1_7C144_Custom_Restore”, else download below:
Start the Download, as you read on: Link
Let’s Roll:
Step 1: Preparing your iTunes
Launch your iTunes9.x (upgrade to latest), connect your iPhone and sync to backup all your data.
Once download of the required .ipsw file completes, Move it to the following location:
Windows 7:
“C:\Users\[username]\AppData\Roaming\Apple Computer\iTunes\iPhone Software Updates\”
Vista
“C:\Users\[username]\AppData\Roaming\Apple Computer\iTunes\iPhone Software Updates\”
XP
“C:\Documents and Settings\[username]\Application Data\Apple Computer\iTunes\iPhone Software Update\”
Now, Re-open iTunes, you will be prompted to install the new firmware 3.1, DO NOT UPDATE from this prompt. Just close this dialog.
Step 2: Restoring
Now, lets restore the 3.1 custom Firmware. To do this, press and hold left “Shift” button on the keyboard and then click on “Restore” (Not “Update”) button in the iTunes and then release the “Shift” button.
New window popsup asking you to locate your custom firmware. After selecting, iTunes will start decompressing and then will install. This is an automatic process, sit back and relax.
When it finishes your iPhone 3GS is unlocked and Fully Jailbroken.
Step 3: Syncing (optional)
Once you are done with the unlocking and jailbreak process. You can now restore all your settings, apps, music, contacts and photos to the newly installed firmware version 3.1 by restoring the backup that you made in Step 1 from iTunes.
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Troubleshooting: Follow this guide if you get any iTunes Errors 16xx (1601,1602, 1603, 1604)