How To Unlock iOS 4.3.1 on iPhone 4, 3GS with Ultrasn0w 1.2.1

UltraSn0w 1.2.1 has been fixed to unlock iOS 4.3.1 on previously unlockable Basebands. These are the steps to unlock iOS 4.3.1 with UltraSn0w on iPhone 4 and iPhone 3GS. Before unlocking iOS 4.3.1 on iPhone 4 and iPhone 3GS, read the important part first.

We’ve pushed out the 4.3.1 compatibility fix for ultrasn0w in Cydia — it’s now at version 1.2.1. If you’re not already at 4.3.1 and you need the unlock, please be sure you understand how to get to 4.3.1 using a custom IPSW that doesn’t update your baseband.
This isn’t a new unlock! It’s to allow those who are already unlocked to use FW 4.3.1. It also fixes the signal bar issue for those who aren’t using the unlock but retain an older baseband intentionally.
AFTER INSTALLING ULTRASN0W 1.2.1, PLEASE REBOOT YOUR iPHONE using the normal “slide to power off” swipe. T-Mobile users in the USA also should disable 3G mode in Settings->General->Network.

Note: Ultrasn0w is NOT a new unlock for iPhones with newer basebands, it only fixes the compatibility and signal bar issues on the existing devices, which are jailbroken (compatible basebands: 01.59.00 / 04.26.08 / 05.11.07 / 05.12.01 / 05.13.04 / 06.15.00) on iOS 4.3.1 using PwnageTool or Sn0wbreeze.

iPhone 3GS Only: How to Update iPhone 3GS to 06.15.00 Baseband to Unlock Ultrasn0w 1.2.x unlocking requires your iPhone 3GS is to be jailbroken with 06.15.00 baseband.
Follow the complete guide posted here to jailbreak your iPhone 3GS on iOS 4.3.1 before proceeding with the following steps.

How to Unlock iOS 4.3.1 with UltraSn0w

Step 1: First jailbreak your iPhone 4 or 3GS on iOS 4.3.1 and preserve the old baseband using either PwnageTool on Mac, or Sn0wbreeze on Windows. 

Step 2: Now unlock your iPhone on the supported basebands using Ultrasn0w 1.2.1 from Cydia. To do this:

• Start “Cydia” on your iPhone.
  
• Touch on “Manage” tab on the bottom.
  
• Now touch on “Sources” as seen in the screenshot below:
  

• Touch on “Edit” and then on “Add”. You will be prompted to enter a url source as seen in the screenshot below. Type “http://repo666.ultrasn0w.com” and touch on“Add Source” to add this repo if you haven’t already.

Cydia will now automatically update your sources by following a series of automated steps.

• After installation is completed. Search for “ultrasn0w 1.2.1” in Cydia and install this application. This app will automatically unlock your iPhone so that you can use it with any carrier.

Step 3: Now simply restart your iPhone and you should now have a fully unlocked iPhone 4 or 3GS, running iOS 4.3.1

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