Google Pixel 4a biometric unlock is not working

The checks in more detail

Take off a case that covers or sits too close to the sensor, because a case with a misaligned cut-out presses on it or blocks it. Make sure that the finger covers the whole sensor. Open Settings, Security and privacy, Device unlock, Fingerprint Unlock, enter the PIN and review the saved prints. Delete a print that fails and add it again, lifting and replacing your finger as the screen directs so that the phone sees the edges as well, and register the same finger twice and a second finger for backup. A registration from 2020 may be worn, and skin changes. Check that Unlock your phone is on. A phone that asks for the PIN after a restart or several failed attempts is behaving normally.

The 4a has reached the end of its software support, with no new updates, so a fault that began after the last update is unlikely to be fixed by Google. A sensor that does not respond at all after cleaning and a registration may have failed, perhaps after a fall or liquid damage, and a repair shop can replace it, though the cost may approach the value of the phone. Back up with Google One, restart the phone and test in Safe mode to rule out an app. A battery that has begun to swell can press on the back panel, so check that the phone sits flat.

Keep the PIN safe, since it is the fallback. Because the 4a's sensor is on the back, the finger lands where the camera module sits, and a thick case can move the cut-out slightly, so the finger misses the center of the sensor. Pixel owners describe good results by registering the finger in the position it naturally falls when the phone is picked up, not flat as the setup suggests. Google's Pixel Help page for this phone also notes that Fingerprint Unlock needs a screen lock set first, so check that a PIN is present. After the software support ended in 2023, bugs will not be patched, so a case change or a re-registration is the cure.