How To Unlock iOS 4.3.1 on iPhone 4 / iPhone 3GS using Ultrasn0w 1.2.1

iPhone Dev Team has released an updated version of Ultrasn0w 1.2.1 unlock to fix issues with iOS 4.3.1 firmware. This update is only for 4.3.1 compatibility and ONLY unlocks the baseband that Ultrasn0w 1.2 currently supports.

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.

Ultrasn0w 1.2.1 Supported Basebands

iPhone 4

The official ultrasn0w 1.2.1 repo http://repo666.ultrasn0w.com will cover iPhone 4 baseband 01.59.00, iPhone 3GS basebands 04.26.08, 05.11.07, 05.12.01, 05.13.04 including 06.15.00. Baseband 05.14.02 and 05.15.04 will be still locked and you need to update your iPhone 3GS to baseband 06.15.00. However, you will lose your GPS (known issue) if you install the iPad (6.15.00) baseband. Currently, there is no unlocking for iPhone 4 baseband 2.10.01, 3.10.01, 4.10.01.  If it is clear, you can now follow the guide below on how to unlock iPhone 4, iPhone 3GS on iOS 4.3.1 firmware.

Step 1: Follow the guide below to jailbreak your iPhone 4 or iPhone 3GS riding on firmware iOS 4.3.1

• How to Jailbreak iOS 4.3.1 with Redsn0w 0.9.6
• How to Jailbreak iOS 4.3.1 with PwnageTool 4.3
• How to Jailbreak iOS 4.3.1 with Sn0wbreeze 2.5
• FIX iOS 4.3.1 Jailbreak and Unlock issues – Redsn0w 0.9.6rc9 and PwnageTool 4.3

Step 2: Start Cydia on your jailbroken iPhone iOS 4.3.1 springboard.
Step 3: Select Manage tab at the bottom of your iPhone screen and then select Sources.

Step 4: Select Edit and then select Add. Now enter a URL source. Type repo666.ultrasn0w.com and select on Add Source. Once Cydia has added the source click on Return to Cydia button.

Step 5: Once the installation is completed. Search for ultrasn0w in Cydia and install it.

Step 6: This will automatically unlock your jailbroken iPhone 3GS, 3G running baseband 01.59.00.
Step 7: Reboot it. That’s it!

If you face any trouble while jailbreaking your device, just drop a message in the comment section. Alternatively, you can follow Jeffry McDowell, the author of this post, on twitter and facebook fanpage.  If you’d like to contact him, twitter is the most effective means of doing so.