Previously we shared a method which allowed you to downgrade your iPhone and iPod Touch firmware from iOS 4.0.2 to iOS 4.0.1, however the procedure for downgrading was valid only for those devices who have already saved the SHSH blobs of iOS 4.0.1. Folks at PwnMyI Forums have managed to downgrade iOS 4.0.2 to iOS 4.0.1 on iPhone 4 and iPhone 3GS (New bootrom) even if you haven't saved the SHSH blobs of iOS 4.0.1.

Steps to Downgrade from iOS 4.0.2 to iOS 4.0.1 Without SHSH Blobs:

1. Download iOS 4.0.1 official IPSW file from here
2. Rename the iOS 4.0.1 file from .IPSW to .ZIP or .RAR and extract it on your desktop using WinZip or WinRAR.




3. Open buildmanifest.plist with Notepad on Windows (Use TextWrangler on Mac)
4. Do search and replace all for 8A306 with 8A400 and save the file.



5. Do the same with restore.plist



6. Download iOS 4.0.2 official IPSW from here and change its extension to .ZIP or .RAR. Once changed, open it with WinRAR or WinZip (DO NOT EXTRACT IT, JUST OPEN IT BY WinRAR)




7. Now drag all the files you extracted in step 2 to the iOS 4.0.2 archive you just opened in the previous step.



8. Now delete all the DMG files that have 002 at the end, leaving only the 001 files left and save the iOS 4.0.2 archive.



9. Chance its extension back from .ZIP or .RAR to .IPSW.



10. Add the 74.208.10.249 gs.apple.com line to the host file.



11. Connect your iPhone in DFU mode. To do that just hold "Power" and the "Home" button for exactly 10 seconds and then release "Power" button but keep holding "Home" until iTunes detect your iPhone in Recovery Mode.
12. On the summary page, hold down the SHIFT key on keyboard and press Restore button. You will be asked to choose the firmware file you want to restore, choose the modified iOS 4.0.2 IPSW file. If you are a Mac OS X user, use the ALT+Restore combo to choose the firmware file.



13. Now you can jailbreak iOS 4.0.1 with JailbreakMe and unlock it with Ultrasn0w.

[via PwnMyI Forums]