Reformat / reset an unrooted phone and wipe /system I'd like to reformat / reset my verizon Samsung S4 (SCH-I545, baseband **OF1, Android 5.0.1). After doing a number of tests and experimentation, I believe that there is a malicious binary sitting in /system which is causing me to follow a random Russian porn Instagram account. I'd like to get rid of whatever is causing the auto-follow (it's not even good porn), so I thought a hard reset / reformat would be ideal... HOWEVER, I believe that a factory reset does not touch /system. Is there a recommended approach with a non-rooted phone to essentially start with a blank slate?

The phone itself was never rooted, but it did come refurbished a year or so ago from Verizon. Since it's running a OF1 baseband, I believe rooting it is out of the question? If it's possible to root, that might solve a lot of problems.
The easiest way to fix it (without even voiding warranty because we will be flashing an official firmware image) consists of a few small and easy steps:

(Assuming you have already installed Samsung's USB drivers)
I take no responsibility for damaging, bricking or otherwise damaging you or your phone, but that's how I flashed since a few years now and it never failed me, even when I was at the glimpse of a hard-brick.


Get your firmware image: SamMobile has you covered on nearly all the Samsung firmware images. Get the latest image from Sammobile for your device by creating an account and downloading the .tar.md5 file. (Don't change the extension, even though you can open the .tar like usual, it prevents Odin from verifying the checksums)
Get Odin from Sammobile.
Boot your phone to download mode by shutting it down and then holding HOME + Volume-Down + Power until you are greeted by the bootloader. Then press Volume-Up to confirm that you want to install a firmware.
Open Odin and select the .tar.md5 as PDA or AP, leave everything else untouched (uncheck Re-Partition just in case! It can mess with your storage layout and is usually not needed)
Connect your phone to your PC (if you haven't already) and make sure it's recognized by Odin.
Hit the Start button
Wait until your phone automatically reboots and you are done. No factory reset is needed, the /system partition will be checksum-exact with the one in the firmware image.


(All images with a solid, non-grayed out KIES logo are exact with the one available form Samsung's KIES software)