It worked every time for a year and now it fails half the time, or only in the morning, or never in the car. Face unlock is more sensitive to conditions than people expect, and the causes of a sudden decline are usually simple and fixable.
The short answer
Clean the front sensor area first, because a smudged or partly covered sensor is the most common cause and takes ten seconds to rule out. Then re-register your face, ideally adding an alternate appearance for glasses or a beard. If it fails specifically in bright sunlight or total darkness, that is a limitation of the sensor rather than a fault.
Two different technologies
This matters because they fail differently:
- Camera-based recognition uses the ordinary front camera. It needs reasonable light, is fooled more easily, and is usually offered as a convenience rather than a high-security method. It struggles in the dark and in strong backlight.
- Depth-sensing recognition projects an infrared pattern and builds a three-dimensional map. It works in complete darkness and is far more secure, but it is sensitive to anything blocking the infrared emitter and to very strong sunlight, which swamps the infrared pattern.
If your phone unlocks in a dark room, you have the depth-sensing kind. If it does not, you have the camera kind, and failing in low light is expected behaviour.
Clean the sensor area
Wipe the entire top bezel or notch area with a microfibre cloth, not just the obvious camera lens. The infrared emitter and receiver are separate small windows, and a fingerprint over one is enough to cause failures while leaving the camera looking clean.
Check for a screen protector covering the sensors
This is a very common cause. A protector with a slightly wrong cutout, or one that covers the infrared windows with material that blocks infrared even though it looks transparent, breaks depth sensing entirely. Test with the protector peeled back at the top.
Re-register your face
Delete the stored face data and set it up again, in even indoor light, holding the phone at the distance and angle you normally use. Appearance drifts over months and a fresh registration often restores reliability immediately.
Add an alternate appearance
Most phones support a second registration for glasses, a hat, a beard, or make-up. If you regularly wear something the original registration did not include, add it rather than hoping the system adapts.
Check the attention requirement
Many phones require open eyes looking at the screen. That is a security feature and it is why face unlock fails when you glance sideways or wear certain sunglasses. You can usually turn it off, at some cost to security.
Test in different lighting
Direct sunlight overwhelms infrared sensors; complete darkness defeats camera-based ones. Note where it fails, because a consistent pattern points at the technology rather than a fault.
Restart the phone
A stuck biometric service produces repeated failures with no other symptom, and a restart clears it.
Check for updates
Face recognition algorithms are updated in system software, and there have been releases that improved reliability noticeably.
The screen protector cause deserves emphasis because it is so easy to miss. Some protectors use an infrared-blocking coating that is completely invisible to the eye. If face unlock stopped working the day you fitted a new protector, that is your answer and no amount of re-registering will fix it.
Why it fails in specific situations
- In the car. Strong sunlight through the windscreen swamps the infrared pattern, and the angle is often wrong.
- First thing in the morning. Your face genuinely looks different: puffiness, no glasses, hair down. Adding an alternate appearance helps.
- Lying down. The phone is at an unusual angle relative to your face. Some phones handle this and some do not.
- Wearing sunglasses. Many block infrared, which defeats the attention check and sometimes the recognition itself.
- Outdoors in bright sun. The most common environmental failure for depth-sensing systems.
- With a mask or scarf. Depends on the phone; some support partial-face recognition and many do not.
Always keep a working PIN or password and make sure you remember it. Biometrics fail, and phones deliberately require the passcode after a restart, after several failed attempts, and periodically regardless. A phone you cannot unlock because you have forgotten the passcode usually means a factory reset.
If the hardware has failed
If face unlock stopped working entirely rather than becoming unreliable, and cleaning and re-registration have not helped, the sensor may have failed. This is most likely after a drop or a screen replacement, since the sensor array sits at the top of the display assembly and is easily disturbed.
Worth knowing: on some phones the depth-sensing hardware is paired to the specific device at manufacture, which means a third-party repair can leave face unlock permanently disabled even when the part is genuine. If face unlock matters to you, ask about that before authorising a screen repair.
Why did face unlock stop working after I fitted a screen protector?
Many protectors cover or block the infrared sensor windows, sometimes with a coating invisible to the eye. Peel the top of the protector back as a test before anything else.
Why does it fail in bright sunlight?
Strong sunlight contains a lot of infrared, which swamps the pattern the sensor projects. It is a limitation of the technology rather than a fault with your phone.
Should I re-register my face periodically?
It helps if your appearance has changed, or if reliability has drifted over months. Adding an alternate appearance for glasses or a beard is usually more effective than re-registering the same look.
Is face unlock as secure as a fingerprint?
Depth-sensing systems are broadly comparable. Simple camera-based recognition is considerably weaker and is better thought of as a convenience than a security measure.
Can a screen repair break face unlock?
On some phones, yes, permanently, because the sensor is paired to the device at manufacture. Ask about this before authorising a third-party screen replacement if face unlock matters to you.
The bottom line
Clean the whole sensor area and check your screen protector is not covering it, because those two explain most sudden failures. Re-register your face and add an alternate appearance for glasses or a beard. Failures in direct sunlight or total darkness are the technology’s limits rather than a fault, and keeping your PIN memorised matters more than any of it.
