Short answer: a correctly fitted protector does not stop Face ID and does not stop an in-display fingerprint reader. What stops both is an air gap, not a thickness — and the fix for a fingerprint reader that has gone unreliable is almost always to re-register the finger.
Face ID reads through a hole, not through glass
The TrueDepth array projects and reads infrared through the cutout, and the cutout is cut clear rather than covered. There is nothing in the light path to interfere with. A sheet whose cutout is the wrong shape for the phone is a different problem — that is a fit failure, not a Face ID failure.
The reader is the one that actually cares
An in-display reader has to get a signal through whatever you put on the panel. Optical readers photograph the ridges of your finger with light from the display itself. Ultrasonic readers send a pulse and listen for the echo. Both work through 0.33mm of glass without difficulty — as long as the glass is bonded to the panel with nothing between.
Air is the problem. An unbonded pocket scatters light and reflects sound, and a reader that was reading a finger is now reading an interface it was not designed for. This is why a protector with a bubble over the reader fails while an identical protector with none works perfectly.
Thickness is not the variable people think it is. A 0.2mm sheet with an air gap over the reader performs worse than a 0.33mm sheet that is properly wetted out.
Full-glue is why it works
Adhesive across the whole face, not just a frame around the edge, is what removes the air. A perimeter-glue sheet leaves the middle of the panel — where the reader is — unbonded by design. Those are cheaper to make and they are the ones that generate fingerprint complaints.
Re-register after you fit it
Do this even when the reader seems fine. Your phone stored a model of your fingerprint as seen through bare glass; it is now seeing it through bare glass plus 0.33mm and an adhesive layer. The image is slightly different. Delete the finger and add it again, and take the time to roll the finger through the angles the setup asks for.
Most reports of a reader that got worse after fitting resolve here, in under a minute, and never come back.
When to suspect the sheet
If you have re-registered and it is still unreliable, look at the panel over the reader at a shallow angle against a light. If there is a visible pocket there, lift the corner nearest it, clear the particle with a dust sticker and lay it down again. If the pocket is clear and it still fails, the sheet is wrong for that phone.
Questions people ask
Does a tempered glass protector stop Face ID working?
Not when it is cut for your phone. Face ID reads infrared through the notch or Dynamic Island cut-out, and that cut-out is left fully clear, so nothing sits in its light path. If Face ID fails with a protector on, the cut-out is the wrong shape for the phone.
Why did my fingerprint sensor stop working after fitting tempered glass?
Usually the phone just needs your fingerprint again: it stored your print as read through bare glass. Delete the finger and add it again. If it still fails, look for a pocket of air or dust over the reader — an air gap is what upsets an in-display reader, not the thickness of the glass.
Does tempered glass work with an ultrasonic fingerprint reader?
Yes, as long as the glass is bonded to the panel with nothing in between. Optical and ultrasonic readers both work through 0.33mm of glass; an air gap is what reflects an ultrasonic pulse and scatters an optical reader’s light.
What is full glue tempered glass?
Glass with adhesive across its whole face, not just a frame round the edge. A perimeter-glue sheet leaves the middle of the screen — where an in-display reader sits — over an air gap. Full glue removes that gap, which is what keeps the reader and touch working as before.
Does thicker tempered glass affect the fingerprint reader?
Less than the air gap does. A 0.2mm sheet with air over the reader performs worse than a 0.33mm sheet that is properly bonded.