How To Guide G998U and G998U1 Firmware Links and Flashing Discussion Here

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Mar 4, 2022
Samsung Galaxy S21 Ultra
Well... the only other thing I can suggest is get the Samsung Band Selection app and see what bands you have available. Also use it to check the "More Network Settings" to see if everything is good there. This app gives you a few more Settings than what are available in the main phone settings menu, so it's possible that something is turned off that shouldn't be. Good luck!


Senior Member
Apr 19, 2017
That would certainly seem to indicate that it's still Verizon locked. It definitely should have worked with that card.
It was a hardware issue. I ordered a different S21 Ultra just to see if it would work and it accepted my SIM immediately. Same exact ATT/ATT,ATT/VZW service carrier as the original one, but for whatever reason that one wouldn't accept Verizon or Visible SIMs.


Mar 4, 2022
Samsung Galaxy S21 Ultra
It was a hardware issue. I ordered a different S21 Ultra just to see if it would work and it accepted my SIM immediately. Same exact ATT/ATT,ATT/VZW service carrier as the original one, but for whatever reason that one wouldn't accept Verizon or Visible SIMs.
Hmm, that doesn't seem to really make any sense if it works on ATT and TMB, but guess it doesn't really matter if you found a solution. Glad that you figured something out


Senior Member
Mar 25, 2010
I got the May OTA today on my G998U1, looks to be build CVDC.



New member
May 6, 2022

I need some help a flashed U1 firmware xaa to my galaxy s21 ultra. It was previusly locked to at&t with u firmware. In instructions says you have to insert a sim card of the carrier whose firmware you want upon boot but my sim card slot isnt working i can only use esim. Does anyone have any sulution? Thanks


Senior Member
Feb 7, 2011
Has anyone successfully flashed U1 on Hong Kong or Taiwan TGY or BRI variant? I'm using a taiwan phone with ATT and not getting wifi calling or 5g

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    For those who aren't already aware, G998U is the model number of all of the US carrier versions of the S21 Ultra. The G998U1 is the model number of the same device, but sold by Samsung SIM unlocked and with no carrier bloatware. Firmware is 100% interchangeable among these models: any U or U1 firmware version can be flashed to any U or U1 device at any time, assuming that you're not trying to downgrade the bootloader. A "U to U" or "U1 to U1" flash does not require a factory reset, but changing from one firmware to the other will require one. Also, U firmware is identical for the 4 base files (AP, BL, CP, CSC) for each individual build, no matter which carrier it comes from. So if you download 2 different carrier versions of the exact same build, the 4 base files will be identical. Where the firmware differs is in the USERDATA file - that's where the carrier bloatware and customizations reside.

    Manual Odin flashing is designed for those who:

    - want to manually update to a newer version of U firmware on their U device, either because they don't want to wait for their carrier's OTA or because OTAs aren't working on their device, or a different carrier got a new feature/patch before they did
    - want to flash U1 firmware to their U device
    - want to convert from U1 firmware to carrier U firmware
    - want to convert from one carrier's U firmware (with all apps and bloatware) to a different carrier's U firmware (with all apps and bloatware)

    For those who aren't already aware, anyone can now download firmware for almost any Samsung device any time that they want, via the various firmware download tools posted here on XDA like SamFirm or Frija. There are threads and info about them here on XDA so I won't waste time explaining - do a search if you're not already familiar with these tools. But they only provide whatever the latest/newest/current version is - nothing older. I pay for a subscription to a site that gets the official releases (usually) before they hit the firmware download tools, so that's where my files come from.

    This is not a "beginner's guide to Odin flashing" -type post/thread, so I won't list basic, step-by-step flashing instructions. My threads are geared towards those who already know what they're doing. If you're new to manually flashing Samsung firmware (or haven't done it in a long time), I recommend you consult one of those "beginner's" guides- there are tons of them here on XDA.

    Happy flashing!

    U firmware:
    U1 firmware:



    - I have lots of threads just like this one for other Samsung flagships, if it interests you. For those who have already made use of my previous threads for a different device and are upgrading to an S21, welcome back :)

    - My zips are compressed extra small for space and bandwidth savings. The default Windows extractor can't handle it, so you'll need a 3rd party tool like 7zip, WinRAR, etc

    - it's quite time consuming to download the files, zip them up, and upload them to AFH. And, I do threads like this for every single Sx and Note x model, so I won't be uploading every single version. The only full builds that I upload are ones where I get access to a new bootloader and/or Android version early, before they hit the firmware download tools.

    - My naming method for folders on AFH is: build - bootloader - Android version
    So "SAT - 1 - 9.0" means the full build number ends in SAT, it's bootloader version 1, and it's Android 9.0. When bootloader and Android versions increment, the first build to have those changes will have its folder notated appropriately.

    - HOME CSC file keeps data intact; using the other CSC will wipe data. Flashing USERDATA will also wipe data.

    - Since AT&T doesn't allow their firmware to be publicly available (for any of their devices), I'll try to always upload at least one USERDATA per bootloader increment for AT&T (and Cricket, if they get this model). I'll also upload any full builds if AT&T is the only carrier to get that specific build, because you won't be able to find the files anywhere else.

    - For identifying USERDATAS, and to know what region/CSC to use in the firmware download tools if you're downloading yourself:

    AIO = Cricket (not available via firmware download tools)
    ATT = AT&T (not available via firmware download tools)
    BST = Boost Mobile
    CCT = Xfinity Mobile (Comcast)
    CHA = Spectrum Mobile (Charter)
    DSH = Dish Network
    SPR = Sprint
    TMB = T-Mobile
    TMK = Metro PCS
    USC = US Cellular
    VZW = Verizon (available via firmware download tools, but Manual Entry is required - Auto will not work)
    XAA = U1 firmware

    - the correct procedure for switching from one carrier's firmware to another carrier's firmware is:
    1. Flash full U1 firmware package including non HOME CSC
    2. Upon boot, insert a SIM of the carrier whose firmware you want
    3. Watch for the popup that says "reboot to apply new carrier settings" etc, and do it
    4. After that restart completes, reboot back to Download mode and flash the full U firmware package, with the USERDATA file of the carrier whose firmware you want
    Initial U firmware and AT&T USERDATA are posted, along with U1 firmware. U firmware is not yet accessible in the firmware download tools like SamFirm and Frija, but if anyone wants a different carrier's USERDATA (for flashing bloatware to a U1 device or for converting a U device from one carrier's bloatware to a different one), let me know and I'll get it for you
    I've seen nothing about the May update for the G998U1. Older phones are getting it but we haven't.
    Have the Verizon version, what is the benefits of flashing to U1?
    The only benefits I can think of would be enabling Samsung features that Verizon disables, like the Spam Filter, and removing some of the bloat that might be impossible to remove on the VZ version.
    I would think you would have most features since the U1 firmware is sold by Samsung to work on networks like T-Mobile. However, all I can verify for you is that I got my wifi calling and I have a 5G icon with 305Mb on speed test. This is after flashing U1.

    I had the same issue. Out of the box, without even turning the phone on, I flashed the U1 firmware on my T-mobile S21 ultra but the device just kept saying "NO SIM"

    If you have a T-mobile device, turn it on with an active sim card first with the original U firmware. Don't need to go through the setup, just have to let it find a LTE/5G signal. After that you can power down and flash U1 firmware and won't have any SIM issues.

    Hope this might help anyone who might run into the same issues we did