The fingerprint sensor is a separate ribbon cable and, unfortunately on this model, it is not secured in any way other than friction. That cable disconnecting is one of the most common issues with this device.[br]
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Luckily, it is one of the easiest to fix, as well. It requires only a minute and no specialty tools. Of course, you can always use iFixit opening tools and picks to make some steps easier.[br]
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Before working on your phone, always turn it off.[br]
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Remove the sim card. If you do not have an ejection pin, a paperclip will do. Pop off the back of the phone (there is no adhesive and you can do so with just your fingernails). It is snapped on with pressure clips that release easily. You will have to go around all 4 sides and it may take a bit if you are unsure. Just be gentle and take your time.[br]
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Lift the phone back gently; with the glass facing down and the top of the phone upwards, open it like a book written in English where you pretend the left side is a hinge and open from the right side, laying the back down next to the phone on the left.[br]
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There should be a ribbon cable attached to the back. it is on the right of the back. If it is connected to the phone, find where it is connected and disconnect it. This requires nothing more than gently prying with a fingernail.[br]
If it is not connected to the phone, you'll see on the rest of the phone a plastic frame over the motherboard (top side, above battery), and right below the cameras, a small square in the plastic frame that has circuit board exposed and a press connector.
Find the fitting on the end of the ribbon cable from the back of the phone, and connect it again to the press connector, making sure it is seated firmly.[br]
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Put the back on again, making sure to get all the snaps connected on all sides, and turn the phone back on. Test your fingerprint reader. If it was this very common issue, congratulations, you fixed it! If that did not solve it, the problem is likely a faulty reader or a damaged connector or ribbon cable, which can usually (though not always) be fixed by replacing the whole reader/cable assembly that is attached to the back of the phone. But definitely attempt connecting/reconnecting it first, as that happens to this particular model way too often.