So want to downgrade 3G to 3.1.3 firmware. The firmware on the 3G is indeed the ideal, running more handy. Following describes how downgrade iPhone 3G to 3.1.3 firmware. This tutorial applies to models: iPhone3G all versions, whatever the firmware version and baseband.
The steps of downgrading iPhone 3G iOS 4.2.1 to iOS 3.1.3:
1. With your downloaded RedSn0w to guide iPhone 3G into DFU mode.
2. Then open your iTunes, it will immediately pop up a dialog box that prompts an iPhone can be restored.
3. This time you need hold down the Shift and the restore button on iTunes.
4. Select your downloaded the official 3.1.3 firmware.
5. iTunes to verify youre iPhone 3G and install.
6. In the last iTunes will prompt an error 1015, this time need run the downloaded RedSn0w (iTunes will automatically exit when running RedSn0w).
7. Here we click on Browse to select our official download firmware 3.1.2 jailbreak, we only select to install Cydia then the normal operation of the jailbreak with RedSn0w.
8. The jailbreak 3G will stuck when a large pineapple running to 4/3, this time we directly hold the power button + Home button and then the phone into DFU mode with a black screen, then turn off the RedSn0w.
9. Once again, repeat the above operation to run RedSn0w, this time we can see the phone quickly and ran over a large pineapple, then you can enter to the iPhone home screen.
10. But such access to the desktop we will find the Cydia does not work.
11. Then we need use blackra1n Spirit to guide the phone, directly downloadedblackra1n, this which only one button, press down, it will automatically repair the phone, then will auto reboot.
The tools need to be prepared:
1. If you have a WiFi environment, directly run Cydia on your phone desktop, the first running to refresh the data, then click the second bottom button right corner which likes a “book”, and then choose the middle of the “application library”. This is inside a ultrasn0w source “repo666.ultrasn0w.com”, click into and install the latest ultrasn0w to unlock your iPhone 3G baseband besides 05.14,05.15.
Using ifunbox uploaded the unlock tool to path: “iPhone\private\var\root\Media\Cydia\AutoInstall“, first upload the mobilesubstrate_0.9.3367-1_iphoneos-arm.deb, then manually reboot iPhone (repeat twice), then add the ultrasn0w_1.2.3, reboot twice again, then set the iPhone 3G Settings – General – Network – Enable 3G (shut off) and then wait to capture the 3G signal.