[How To] Restore iPhone 3G/3GS To iOS 4.2.1/4.1 On BB 06.15.00, After Using Ultrasn0w 1.2

In case you have unlocked your iPhone 3G/3GS by flashing iPad baseband 6.15.00 with ultrasn0w 1.2 on iOS 4.2.1, and have issues while restoring your device, follow the simple guide posted by Sherif Hashim, the developer behind ultrasn0w unlock tool, showing how to restore your iPhone 3G/3GS and resolve all the issues.
Note:
To restore your iPhone 3G/GS from 4.2.1 to 4.1, you must have your SHSH blobs saved for iOS 4.1.

How To Downgrade iPhone 3G/3GS on iOS 4.2.1 (BB 06.15.00) to iOS 4.1:

posted by sherif_hashim:

To restore ur iphone while already on 6.15.00 BB:

A) if u used pwnage tool then u ll have to create a /new/ custom 4.1 or 4.2.1 and say /no/ when u are offered the BB update to 6.15.00 since u already have it, then build the custom ipsw and restore to that

B) if u r using redsn0w, then:

B-1) if u are currently on 4.1, then u can restore to official 4.1 again, and when u see error 1015 pop-up, u kick ur device out of recovery using umbrella, use redsn0w and /don't/ check "ipad BB update" because u already have that :)

B-2) if u are currently on 4.2.1 then u ll have to restore back to 4.1 (because 4.2.1 will not allow u to kick the phone out of recovery when 1015 occurs due to the silly BB / iOS mismatch check added there) , then do the same steps as B-1, if u want to go again to 4.2.1 your only way is to use a custom 4.2.1 done by pwnage tool in the same steps mentioned in A

B of course saved shsh for the 4.1 ipsw is an important prerequisite for iphone 3gs to restore back to that

UPDATED
 

For PwnageTool users:

A : If you are on Mac and use PwnageTool: If you used pwnage tool then you will have to create a “new” custom 4.1 or 4.2.1 and say “No” when you are offered the BB update to 6.15.00 since you already have it, then build the custom ipsw and restore to that.

For Redsn0w users:

B : If you use(d) Redsn0w:

B.1: If you are currently on 4.1, then you can restore to official 4.1 again, and when you see error 1015 pop-up, you kick your device out of recovery mode using TinyUmbrella, use redsn0w and don’t check “ipad BB update” because you already have that Here Is How To Restore iPhone 3G And iPhone 3GS While On Baseband 06.15.00 And Having Issues icon smile

B.2: If you are currently on 4.2.1 then you’ll have to restore back to 4.1 (because 4.2.1 will not allow you to kick the phone out of recovery when 1015 occurs due to the silly BB / iOS mismatch check added there) , then do the same steps as CASE-1.

How to upgrade to iOS 4.2.1:

If you want to go again to 4.2.1 your only way is to use a custom 4.2.1 done by pwnage tool in the same steps mentioned in A .

Try this methods listed above to restore your device between 4.2.1 and 4.1, it’s worth trying. And, don’t forget to tell us about your experience in the comments section below