Have you updated your device recently? If not try doing that, and after leave your battery out for 10s. Once rebooted try using an application with them. Also where did you purchase your device from, if it wasn't directly from a trusted store, you may have a knockoff.
Goodluck :)
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=== Update (11/11/2017) ===
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You can test your sensors via: Phone app> Dial: *#0*# > Klik Sensor.
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The IP68 certification implies water resistents in the case of fresh none streaming water and a phone in new-state. This does not imply water with chemicals in it(salt water, chloor etc.). This means that the phone as rolled out of the factory is water resistant and can stand fresh water to up to 1.5m.
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When the phone falls into a bucket it might happen that the sealed edges are not sealed anymore because of the fall damage, this can also happen when user-damage is on the phone.
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In most cases water damage is not covered via warenty; if you are confident that what is mentioned above is not the case on your phone you can always send it for repair. But keep in mind the repairman can see in the phone if water damage has slipt because of user damage. If this is the case the costs for repair will be your own.
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(P.S This isn't my answer, Credit goes to userfTPcEJKnSe @ https://us.community.samsung.com/t5/user/viewprofilepage/user-id/8273010 )
Have you updated your device recently? If not try doing that, and after leave your battery out for 10s. Once rebooted try using an application with them. Also where did you purchase your device from, if it wasn't directly from a trusted store, you may have a knockoff.
Goodluck :)