The Galaxy S21 reads fingerprints with an ultrasonic sensor under the display, and a sensor that has been through years of use and a few screen protectors often needs a fresh start. The ultrasonic sensor reads through glass but fails through an air gap, so a protector that is not designed for it, a cracked screen over the sensor or a film with bubbles will stop it working. Clean the screen and the finger and press firmly.
The checks in more detail
Delete and re-enrol your fingerprints, adding the same finger several times, and enrol with the protector you intend to keep. A restart helps after an update. Face recognition on the S21 is a convenience feature and less secure; use the fingerprint or PIN for payments. If the sensor works for a while and then stops, a software glitch is likely and a cache partition wipe may help. The S21's screen can crack at the corners and the sensor sits low in the screen, so a crack over it needs a repair. Use the PIN meanwhile.
Sunscreen, hand cream and very dry skin all change how the fingerprint reads, so enrol under normal conditions and wipe the screen before unlocking. If the phone has a tempered glass protector with a cutout that does not match the sensor, the glass itself may be fine but the adhesive area around the sensor can leave an air gap; reapplying the glass fixes it. Edge cases include a phone with a screen replaced by a third party:
a non-Samsung screen can lack the sensor's calibration and make the fingerprint fail or not work at all, and only a Samsung-authorized repair restores it. If you bought the phone second hand, ask whether the screen was replaced, because that explains a fingerprint that never worked. If the phone vibrates but does not unlock, or shows Fingerprint not recognized repeatedly, wait a moment and try again with the finger flat; many failures come from touching and lifting too quickly. The ultrasonic sensor builds an image over a second, so rest the finger rather than tapping it.
Differential
Likely causes of “Samsung Galaxy S21 biometric unlock is not working”, and how to tell them apart
- 1
A wrong protector or air bubbles
What separates it Unlock stopped after fitting one
How to confirm Remove it and test, then fit a suitable one
- 2
A dirty screen or finger
What separates it Reads fail
How to confirm Clean and dry
- 3
A weak enrolment
What separates it Some touches miss
How to confirm Delete and re-enrol, adding the finger more than once
- 4
A cracked screen over the sensor
What separates it Reads fail at one spot
How to confirm Repair the screen
- 5
A software glitch
What separates it It stopped after an update
How to confirm Restart and wipe the cache partition
Before you touch a setting
Record this first — changing settings destroys the evidence
- The protector type
- Any crack near the sensor
- How many prints are enrolled
- Whether it began after an update
Try in this order
0 of 8 completedClean the screen and your finger
ReversibleWith a soft cloth.
Remove the protector
ReversibleTo test.
Press firmly
ReversibleOn the marked area.
Delete and re-enrol prints
ReversibleSettings, Biometrics, Fingerprints.
Enrol with the protector you will use
ReversibleSo the sensor learns it.
Restart and wipe the cache partition
ReversibleAfter an update.
Enrol with clean, normal hands
ReversibleNot just after hand cream.
Reapply a protector with an air gap
ReversibleTo restore sensor contact.
Model-specific
What is true of this model and not of its siblings
- Ultrasonic
- It reads through glass but not through air.
- Re-enrol
- Enrolling with the protector helps.
- Skin condition
- Cream and dryness affect reads.
- Air gaps
- Adhesive gaps block ultrasound.
Stop boundary
When this stops being a self-service repair
- !If it fails on a bare clean screen after fresh enrolment, contact Samsung
- !If a crack covers the sensor, book a screen repair
- !Use the PIN meanwhile
Evidence ledger
Sources behind this answer
Exact questions
Common questions
Why does my S21 fingerprint fail?+
A protector, dirt or a weak enrolment.
Is the sensor ultrasonic?+
Yes.
Is face unlock secure?+
Less than the fingerprint.
Can a crack stop it?+
Over the sensor, yes.
Why does hand cream stop my fingerprint?+
It changes the skin's texture.
What is an air gap?+
A space under the protector.