Guide to Update T-Mobile Galaxy S3 T999 to Android 5.1.1 CyanogenMod 12.1


Android 5.1.1 Lollipop is now out which means we are seeing some major custom ROMs getting the update too. You can now update your Samsung Galaxy S3 T999 to Android 5.1.1 based CyanogenMod 12.1 custom firmware. While you won't see the official Android 5.1.1 update coming from T-Mobile, there is no reason why you shouldn't update your phone via the custom means. Follow this guide to update Galaxy S3 T999 to CM12.1 Android 5.1.1 Lollipop custom ROM.

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Prerequisites to update Galaxy S3 T999 to CM12.1 Android 5.1.1:

  • Backup any important data from your Samsung Galaxy S3 SGH-T999 as a complete wipe will be required during the flashing process.
  • Enable USB debugging option on your device: Settings > Development Options > USB debugging.
  • Samsung Galaxy S3 T999 must be rooted, unlocked and installed with a recovery tool like TWRP or CWM.
  • Your device must be charged to at least 60% of power.

Note: this tutorial is focused on T-Mobile Samsung Galaxy S3 SGH-T999; please do not try this guide on other AndroidSamsung devices.

How to update Galaxy S3 T999 to CM12.1 Android 5.1.1:

Required files: download CyanogenMod 12.1 Android 5.1.1 custom firmware for Galaxy S3 from here and save it on your computer. Do not extract the contents.

  • Connect Galaxy S3 to your computer using a USB cable and copy the zip file to your phone’s SD card.
  • Disconnect the USB cord and power off your Galaxy S3.
  • Boot it into recovery mode by pressing and holding Volume Up + Home + Power buttons together.
  • Perform a Nandroid backup of your current OS: Backup backup and restore on ClockworkMod recovery.
  • Wipe data task: from CWM recovery menu, select wipe data / factory reset and confirm the process. Then go on to wipe cache partition and wipe dalvik cache.
  • From recovery menu, select install zip from sdcard followed by choose zip from sdcard. Locate the CM12.1 Android 5.1.1 custom firmware and confirm the flashing process.
  • Return back to main recovery menu and reboot your T-Mobile Samsung Galaxy S3 into the new custom ROM. First boot takes a good 5 minutes so please be patient.

How to restore: If you want to return to previous OS on your SGS3, boot into recovery, select backup and restore then restore your previous ROM by selecting it from the list.

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