How to Downgrade iPhone 3G 4.0 OS to 3.1.3 Firmware on Windows & Mac

The features of iPhone OS 4.0 are definitely tempting that most of us want to try iPhone 4.0 now. Unfortunately for general public iPhone OS 4.0 won’t be available until summer 2010. However Apple has made iPhone OS 4.0 dev preview available for iPhone Developers to try and test their applications and the same build has been leaked on internet.

Users are reporting that iPhone OS 4.0 Beta is still buggy and iPhone 3G 4.0 reboots unexpectedly anytime. Those unfortunate users who upgraded their iPhone 3G to 4.0 firmware out of curiosity must be looking for way to downgrade back to 3.1.3 firmware.

So we are here with simple & working guide to downgrade iPhone 3G 4.0 OS to 3.1.3 or lower firmware. One thing you must keep in mind that by following this guide you can only downgrade your firmware but your baseband will not change hence you can’t unlock your iPhone 3G even after successfully downgrading iPhone 3G OS 4 to 3.1.3. Download links are provided at the end of this post.
Downgrade iPhone 3G 4.0 OS to 3.1.3 Firmware

1. Download original iPhone 3G 3.1.3 firmware file.
(http://www.blogsdna.com/6841/download-iphone-os-3-1-3-firmware-for-iphone-3g3gs-ipod-touch.htm)

2. Connect iPhone 3G to computer

3. Now Start iTunes 9.1 and on Windows (Press Alt Key+ Click Restore), on Mac (Hold Option + Click Restore), Now manually select iPhone 3G 3.1.3 firmware.

4. During the restore process you will get an 1015 error. Don’t worry, that’s normal. Just click on Ok and close iTunes.


Now open terminal on Mac/ Command Prompt on Windows and type following commands

cd Desktop

. / iRecovery -s

setenv auto-boot true

saveenv

fsboot

exit


6. Now unplug your iPhone and turn it off by pressing the home+power button.

7. Once it is off, turn it on by pressing the power button. Plug-in your iPhone and load iTunes to continue with the activation.

That’s it we have successfully downgraded iPhone 3G 4.0 to 3.1.3 OS.

Now you can use Blackra1n, PwnageTool 3.1.5 or Redsn0w 0.9.4 to jailbreak iPhone 3G 3.1.3.
Download Links

1. iTunes 9.1 (latest iTunes)
(http://www.blogsdna.com/8753/apple-itunes-9-1-with-ipad-syncing-support-for-windows-mac-released.htm)

2. Stock iPhone 3G 3.1.3 Firmware
(http://appldnld.apple.com.edgesuite.net/content.info.apple.com/iPhone/061-7481.20100202.4orot/iPhone1,1_3.1.3_7E18_Restore.ipsw)

3. iRecovery (Mac)
(http://chronicdev.googlecode.com/files/iRecovery-Mac.zip)

4. LibUSB Library Package (Mac)
(http://www.ellert.se/PKGS/libusb-2009-09-10/10.6/libusb.pkg.tar.gz)

iRrecovery & LibUSB Library Package (Windows)
(http://www.zaksenterprises.com/index.php?option=com_remository&Itemid=84&func=startdown&id=32)
iPhone 3GS users can only downgrade to firmware 3.1.3, unless you saved your ECID files to a earlier version.