Downgrade iPhone 3G 4.0 firmware to 3.1.3 firmware
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Launch redsn0w. Click the Browse button.
Navigate to where you saved the 3.1.2 firmware download from earlier. Select it, and click Open.
If you blink you will miss the lines of text that say: Processing firmware, and IPSW successfully identified. Click the Next button.
A pop up will appear and display several messages quickly.
At this screen you can install Cydia, use verbose boot mode (which just shows what the phone is doing when it boots up), you can also select a custom boot logo to replace the Apple logo, or a custom recovery logo to replace the plug in to iTunes logo.
If you select custom logos, a file browser window will open so that you can load your images.
Make your choices and click Next.
Make sure your iPhone is off, and plugged into your computer with the USB cable. Click the Next button.
Instructions for entering DFU mode will appear. They start right away and go very quickly so try to keep up! First hold down the Power (sleep/wake) button for three seconds.
Keep holding on to the Power button, and press and hold the Home button for 10 seconds.
Continue holding the Home button, and release the Power button.
After a few seconds various messages will appear on the screen. At this point you can let go of the Home button.
Note: If your redsn0w screen hangs at the message Waiting for reboot, then simply unplug the USB cable, then plug it back in.
You will see this graphic on your iPhone.
After a bit the iPhone will reboot (and if you chose a custom boot logo it will appear at this point). Done! will appear in the program. You can click the Finish button to close the program.
You will see a few different messages (Installing base utilities, Replacing kernel, Flashing NOR, Installing bundles, Activating, Syncing file systems, Rebooting) on your iPhone during the next couple minutes.
Step 5.
The iPhone will reboot and be hacktivated. Set up your Wi-Fi connection and then launch Cydia.
Press the Search button and enter ultrasn0w. Install ultrasn0w.
When it finishes installing do not push the reboot button, instead press the Home button. Launch Settings, press General, press Network then slide the Enable 3G switch to OFF.
Reboot the iPhone. You are now back on 3.1.3 firmware, you still have 4.0 modem firmware.
