My fingerprint sensor stopped working after fitting a protector. What now?

This is the most common complaint about screen protectors on modern Android phones, and the fix that works most often costs nothing and takes two minutes.

Re-register your fingerprints

In-display fingerprint readers work through whatever sits on top of the display. When you enrolled your prints, the sensor recorded them as seen through bare glass. Fitting a protector inserts an extra layer that wasn't present during enrolment, and every subsequent read is being matched against a reference that no longer represents what the sensor sees.

Delete the enrolled fingerprints and register them again with the protector already fitted. The sensor then learns the pattern through the new layer, and in the majority of cases the problem simply disappears.

Also turn on the touch-sensitivity setting if your phone has one, variously labelled increase touch sensitivity, glove mode or screen protector mode, usually under Display or Accessibility. It raises the capacitive threshold to compensate for the extra layer and exists precisely for this situation.

If that doesn't fix it, the protector is the problem

There are two sensor technologies and they fail differently.

Optical sensors illuminate the finger and photograph the ridge pattern. They are relatively tolerant of thickness but intolerant of anything that scatters or absorbs light, heavy tint, a matte etched surface, or a privacy louvre layer directly over the sensor.

Ultrasonic sensors, used on Samsung's flagship line among others, emit high-frequency sound and read the reflection. They are far fussier. Ultrasound doesn't cross an air gap well, so any bubble, any incomplete adhesion over the sensor area, or any material with a significantly different acoustic impedance will degrade the read badly. This is why ultrasonic phones are the ones with a reputation for protector problems.

The UV adhesive trap

UV liquid-adhesive kits are a particular hazard on ultrasonic sensors. The adhesive is poured across the display and cured, and if it pools over the sensor area the extra thickness attenuates the signal. Some kits ship with a small sticker to mask the sensor region during application for exactly this reason — use it if provided, and use less adhesive than feels natural.

This failure is also the hardest to undo, because UV-bonded glass isn't straightforwardly removable.

What to buy instead

Look for a listing that explicitly states in-display or ultrasonic fingerprint compatibility. This is one of the few specification claims in the category worth weight, because a brand that has bothered to test it will say so, and a brand that hasn't will stay silent not risk the returns.

Avoid heavily tinted glass, privacy filters and matte finishes over the sensor area on a phone where fingerprint unlock matters to you. Thinner is generally better: a 0.25mm protector interferes less than a 0.5mm anti-shock one.

When it isn't the protector at all

If unlock was already intermittent before you fitted anything, the cause may be dry skin, a damp finger, or a sensor that needs cleaning. Wipe the sensor area, dry your hands and try a fresh enrolment before spending money, and if the phone has had a screen replacement, some third-party panels degrade sensor performance permanently regardless of what is on top.