Pixel fingerprint sensors: ultrasonic since the Pixel 9, optical on the a-series, and the advice flips

• Ultrasonic, under the screen: Pixel 9, 9 Pro and 9 Pro XL; the Pixel 10 series; Pixel 11, 11 Pro and 11 Pro XL.
• Optical, under the screen: Pixel 9a and Pixel 10a; Pixel 8, 8 Pro and 8a, and the under-display Pixels before them.
• In the power button: Pixel 9 Pro Fold and Pixel 11 Pro Fold. A screen protector cannot affect unlocking on these.

If you are not sure which you have, the model name is in Settings, under About phone. The a in the name is what matters most: it marks the models that kept the optical reader.

An ultrasonic reader sends a sound pulse through the glass and reads the echo off the ridges of your finger. It needs an unbroken path, so it is defeated by thickness and by air gaps. Thin glass with adhesive across the whole surface is the fix, and a bubble over the sensor area is enough to cause failed reads.

Privacy films are a common cause of trouble on these phones too, because their layered structure interrupts that path. The mechanism is the same one covered in our Samsung Galaxy privacy guide, which applies to these Pixels almost unchanged.

What is fine: thin, clear tempered glass that says fingerprint unlock works on your exact model. Several makers list one protector for the Pixel 11, 10 and 9 together; that is their compatibility claim, and the Pro XL is usually sold separately.

An optical reader lights your fingertip with the display and photographs the reflection. It does not care much about thickness, because light passes through a slightly thicker sheet perfectly well. What defeats it is anything that dims, tints or scatters the light on the way out and back.

Privacy filters are the worst case: their job is to absorb light that is not travelling straight at you, which is exactly the light the sensor collects. Heavily matte finishes scatter the image it is trying to read; a light matte is usually tolerable. Tinted or blue-light layers reduce and shift the light; mild tints are usually fine, stronger ones are not.

What is fine: ordinary clear tempered glass, including slightly thicker sheets that would cause problems on an ultrasonic Pixel.

The Pixel 9 Pro Fold and Pixel 11 Pro Fold read your fingerprint in the power button, so neither sensor type above applies. Choose glass for the outer screen on fit alone, and keep the factory film on the folding inner screen. Our foldable guide explains why nothing rigid goes on the inner panel.

Whatever you fit, re-enrol your fingerprint afterwards. The phone stored your print as read through bare glass; with a protector on, the path has changed. Delete the saved prints and add them again with the protector in place. This resolves a large share of "the sensor stopped working" reports on both sensor types.

Enrolling the same finger twice as two entries, at slightly different angles, helps too, and helps optical readers most.

Finally, check for a bubble directly over the sensor. It shows up as an unlock that works intermittently rather than not at all, which makes it easy to mistake for a software problem.