(adsbygoogle = window.adsbygoogle || []).push({}); The first thing you need to do is get a copy of the iPhone OS 3.1.3 image.
look around google its pretty easy to find.

Next, download RecBoot (available for Windows and Mac). This handy little utility plays an important role in finishing off the downgrade.
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Step 2: Put Your Phone Into DFU Mode
Dismiss the iTunes alert that told you you're in recovery mode, make sure you've clicked the iPhone under the Devices sidebar in iTunes, then hold Shift and click the Restore button (if you're on OS X, Alt/Opt+Click). iTunes will pop up a window prompting you to choose a file. You want to navigate to the 3.1.3 OS you located or downloaded above. Select that file, and iTunes will start the OS restore process. Expect this to take around 10 minutes or so.

When it's finished, you'll likely receive an error message (error 1015) and your iPhone will boot up with a "Connect to iTunes" screen. This is where the utility you downloaded above, RecBoot, comes in handy. Open RecBoot, and click Exit Recovery Mode. In a few seconds, RecBoot should kick you out of plug-me-into-iTunes recovery mode and you'll boot right into your freshly downgraded 3.1.3.

At this point, you've got a fresh 3.1.3 installation

Thanks for reading

Nice tut but it want go when you not save your Iphone whit cydia

i don't understand what you mean explain?

Great too bad i dont have a iphone :/

Thanks for this. I'm looking to purchase an iphone 3gs on ebay. Will this work with any 3gs iphone i purchase?

Additionally, will this allow me to unlock the phone as well? Thanks in advance.

yeah their is no reason why this should not work.

you can unlock the iphone using cydia.

quick tutorial:

Since you will be using purplesn0w to unlock the iPhone, and since it is an unauthorized app, you would first need to jailbreak your iPhone 3GS. i assume you have already done that or else you can read this online tutorial from RWW on You must login or register to view this content. OS to 3.0. Use You must login or register to view this content. software to perform the jailbreak before using purplesn0w.

Important Note: Back up your current iPhone settings before you start off. Just in case something goes wrong horribly, you must be able to restore the device to factory settings.

1. Disable 3G: Go to “Settings” –> “General” –> “Network” and then tap the slider that reads “Enable 3G” to set it to the OFF position. Exit back to the main screen when finished.

2. Add the Geohot Repository to Cydia: Since you have already jailbroken your iPhone, you should have the Cydia app on your iPhone’s screen. Launch the app and select “Manage” from the set of buttons at the bottom of the screen. Tap “Sources” –> “Edit” –> “Add.” Type in apt.geohot.com and hit “Add Source” when done. Tap the “done” button and exit out of the “Manage” section.

3. Install purplesn0w: Look for “Purplesn0w” using Cydia’s search utility. You’re looking for the package com.geohot.purplesn0w. Once you’ve found that, tap it to see the details. Next, tap the “Install” button and then the “Confirm” button just as if you were installing any other Cydia app. Wait while it installs and watch for the success message output in Cydia. Tap “Return to Cydia” when it’s finished installing and then exit from the Cydia app entirely.

4. Reboot your iPhone: Press the power button until you see the message “slide to power off” appear. Use the slider to power off your device then turn it back on. Once rebooted, your iPhone 3GS will be unlocked!!

Thanks for looking at my thread just ask if any more help is needed.

Thanks for this. When jailbreaking and unlocking a 3GS do I need to be concerned about the phone's baseband?

Also, the 3GS i am looking at is running 4.2.1..... You can't downgrade 4.2.1 to 4.1... can you?

thanks great tut