Android 4.0 is due out soon, and if you're using a custom ROM on your phone, you won't be able to receive your carrier's upcoming over-the-air update to Ice Cream Sandwich unless you revert to your phone's stock Android ROM.
In this article I'll discuss how to revert a Samsung Nexus S to a stock ROM--because if you have a Nexus S, you’ve likely put a custom ROM on it by this point (especially if you followed my instructions on unlocking the Nexus S). If you're using a different phone, note that the general instructions are roughly similar for most Android phones out there, but you'll need to find a stock ROM intended for your specific handset model.
Disclaimer: Messing around with your phone's OS can be dicey stuff, and if you're not careful, you could end up with a pretty paperweight. (Of course, you knew this when you decided to change your phone's ROM to a custom version in the first place.) We are not responsible for any damaged hardware or software arising from this process, so proceed at your own risk.
Be sure to line up everything you'll need to get this operation to work. First, grab a Nandroid backup of the stock Android 2.3 ROM; you can get the Nandroid backup from this Megaupload link (I used it; works fine). Also, plug your phone into your computer, and confirm that the internal storage is mounted.
The Nandroid backup comes as a .zip file; extract it somewhere on your PC. Open it up and go to clockwork, backup, and find a folder called 2010-12-22.21.06.06. The 'clockworkmod' directory structure on the phone is the same, for the record, though that isn't really important for the purposes of this tutorial.
Put this folder in the corresponding folder on your phone. It might have existing backups; you don’t have to worry about those, you can ignore or delete them as you see fit. (Read this entire tutorial before you delete anything, though!) An example of how the typical phone’s 'clockworkmod' folder looks is below:
Once all of that material is copied, make sure to copy anything you might not want to lose from your phone to your PC or the cloud. Reverting to stock will completely wipe your phone, so if you want to keep your pictures, music, videos, and the like, this is your opportunity to move them.
From here you can either shut the phone off and boot it into Recovery (hold the volume-up key and power at boot) or go into Clockwork Mod and choose Reboot into Recovery. If you boot into the Recovery console from Clockwork Mod, allow Root access to ROM Manager and let it reboot. Either way, from inside the Recovery console you will be using the volume-up and volume-down buttons to navigate, and the power button to select.
It’s good practice to make a backup in the Recovery console, and then move the backup from the phone to your computer, before you go through with the transformation. That way, you have it just in case something happens. If you haven’t done so already, now is the time to go into 'Backup and restore' and make a backup. You need to reboot the system after the backup in order to mount the internal storage to your PC again, move the backup there, and go right back into Recovery.
(One note: Samsung phones usually handle tweaks and custom ROMs okay. Bricking your phone in this fashion actually seems pretty difficult if you follow the directions, but stranger things have happened.)
From the Recovery menu, choose Wipe data/factory reset, and then select Wipe cache partition. (You won't go into 'Mounts and partitions' and wipe those--that action isn't necessary.) Once those two items are wiped, go into 'Backup and restore' and restore the aforementioned Nandroid backup.
It takes a few minutes to run in some cases, and it will take an extra couple seconds to boot up, but you should be looking at a clean Android install when it's done:
That’s it! Welcome back to Android 2.3. Once you log back in to Google services, it should have an update ready for you to bring the OS back up to 2.3.6.
You don’t need to have the phone locked in order to get updates from Google. If that’s what you want to do, however, follow my instructions on how to unlock the Nexus S as far as getting ADB installed, and then simply run the following in the command prompt:
cd C:\[place where you installed the Android SDK]
adb reboot bootloader
fastboot oem lock
You now have a completely stock and locked phone, ready to receive Ice Cream Sandwich goodness when Google pushes it out. Congratulations!
If you run into any issues or have any questions, hit us up in the comments.