Only certain iPhone 2G models can be downgraded to iOS 1.0, but all of them can be downgraded to as low as iOS 1.1.4. The following tutorial will show you how.
Requires a Windows XP machine.
Outline: Restore to iOS 1.1.4 twice, run ZiPhoneGUI, and restore to 1.1.4 again.
4) Connect your iPhone 2G to your computer
5) Put your iPhone 2G into DFU recovery mode by:
a) Pressing and holding the power button and home button down for 10 seconds.
b) Then, release the power button while still holding the home button down.
c) You can release the home button once iTunes detects the iPhone 2G is in DFU mode (if not, try again).
6) Then in iTunes, hold the Shift key and press the ‘Restore’ button.
7) Select the iOS 1.1.4 IPSW file you downloaded.
Your iPhone should now be restoring to iOS 1.1.4 (this may take some time). Once your iPhone has finished downgrading to iOS 1.1.4, you’ll need to downgrade to iOS 1.1.4 again. You may see an error message after restoring, which is perfectly normal. If your device isn’t in DFU recovery mode already, do the following steps below, again:
8) Put your iPhone 2G into DFU mode again, by:
a) Pressing and holding the power button and home button down for 10 seconds.
b) Then, release the power button while still holding the home button down.
c) You can release the home button once iTunes detects the iPhone 2G is in DFU mode (if not, try again).
9) Then in iTunes, hold the Shift key and press the ‘Restore’ button.
10) Select the iOS 1.1.4 IPSW file.
Once your iPhone 2G has finished restoring to iOS 1.1.4 for the second time, an error may appear again- which is fine. Ensure the phone is connected to your laptop for the following steps.
11) In the ZiPhone Patched folder you downloaded, open ‘ZiPhoneGUI - patched by kIREmK’.
12) Select ‘Debug Boot’ and ‘iErase (README)’, and deselect anything else.
13) Press the ‘START’ button.
Once this process has finished, your iPhone 2G should be at the ‘Connect to iTunes’ screen. Then, we are going to restore to iOS 1.1.4 for the third and final time.
14) Put your iPhone 2G into DFU mode once again, by:
a) Pressing and holding the power button and home button down for 10 seconds.
b) Then, release the power button while still holding the home button down.
c) You can release the home button once iTunes detects the iPhone 2G is in DFU mode (if not, try again).
15) Then in iTunes, hold the Shift key and press the ‘Restore’ button.
16) Select the iOS 1.1.4 IPSW file.
When your iPhone 2G has finished restoring to iOS 1.1.4, it should be at the activation screen. All you need to do now is activate your iPhone 2G. If you don’t have a SIM card, follow method two of this tutorial
here.
After you have activated your iPhone 2G, your device is now running iOS 1.1.4!
Written and edited by Jonathan Yeung