[Q] Can't unroot my XT910

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Hi Guys I'm a Noob
My phone is XT910 and it has several problem so i'm going to service center.

The problem is; I cannot unroot.
I updated recently after root.
After upgrading, I can't use apps that acquire root authority.
also unrooting is impossible. I mean I cannot remove SU so I cannot go to svc center.
S/W version is 65.3.106.XT910K.KT.en.KR

Is there any solution?
and I think it's impossible, but can I downgrade?
thx for reading this.

have you checked adb shell?

is superuser unable to detect root? is superuser still on your phone after the update?

have you try to reinstall superuser or superuser update fixer from the market?