iPhone's recent 3.1.3 firmware update hasn't really been a success for jailbreak users, quite a lot of issues have been reported while upgrading from firmware 3.1.2. Updating to OS 3.1.3 updates baseband of your iPhone as well, it will be 05.12.01. The latest Modem Firmware 05.12.01 baseband is not downgrade-able. So if you accidentally updated your iPhone to OS 3.1.3 and you want to downgrade/revert it back to OS 3.1.2, this guide will help you to do so. Please note that this tutorial is about downgrading iPhone 3G baseband which is currently on 3.1.3 firmware with 05.12.01 baseband and older bootloader 05.08 (5.8). This won't work with iPhone 3GS.
So if you have an iPhone 3G satisfying mentioned criteria then you can use the latest version of Fuzzyband iPhone app which supports iPhone 3.1.3 to downgrade your 05.12.01 baseband to the older 04.26.08 baseband. Please follow following procedure to do so.
Steps to Downgrade from iPhone 3.1.3 Baseband 05.12.01 to Older 04.26.08 Baseband
- You must jailbreak your iPhone 3G running on firmware 3.1.3 before you may downgrade your baseband version from 05.12.01 to 04.26.08. Please refer to following jailbreak your device: redsn0w 0.9.4, sn0wbreeze or PwnageTool 3.1.5.
- After you have jailbroken your iPhone, Go to Cydia on your iPhone and Search for Fuzzyband and then install this application
- Start Fuzzyband app and touch on Downgrade button to downgrade your Baseband from 05.11.07 to 04.26.08
- Now you can easily unlock your iPhone through Ultrasn0w by following the instructions in the unlock guide posted here.