The downgrade iphone 3G is used to fix the problem after update of the GPS baseband iPad 06.15.00,
or to block the release of the operator of the iPhone 3G.
A good system if you backup by 4.1 SHSH is:
1.Download 4.1 stock firmware from apple.
2.Restore 4.1 directly using iTunes.
3.Backup 4.1 SHSH using TinyUmbella.
4.Make a custom firmware using PwnageTool_4.1.3.
5.Restore 4.1 custom firmware using iTunes.
6.”Exit Recovery” using TinyUmbella if need.
PwnageTool_4.1.3
TinyUmbella 5.00.07
http://cache.firmwareumbrella.com/do…la-5.00.07.pkg
iPhone 4.1 stock firmware
http://appldnld.apple.com/iPhone4/06…7_Restore.ipsw
How to Downgrade iPhone 3GS Baseband 05.12.07 / 05.13.04 / 05.14.02 / 05.15.04 / 05.16.02 / 06.15.00