These days I recommended many people to do it downgrade of firmware to be able to switch to iOS 4.0, so many encountered error 1015 when they restored to an older firmware. Error 1015 in the downgrade process is perfectly normal and means that the downgrade process was successful and you installed the firmware you want to downgrade to. Error 1015 appears because your baseband is higher than that of the old firmware to which you are restoring, so iTunes cannot downgrade it and will give an error.
After you receive error 1015 in iTunes, you can jailbreak your phone:
- for firmware 3.1.2 you can use redsn0w ori blackra1n;
- for firmware 3.1.3 you can use Spirit but only if you have the phone operator's card on which the phone is coded to activate the phone.
I hope I have clarified everything about error 1015.
