This is an old topic so I thought it would be good to wrap it up with the following summary. I’m marking this post as “Solution” so that the topic is seen as solved, but it’s the cumulated experience of all the contributors to whom thanks. The information here concerns FP3 standard Fairphone Android (stock OS).
- Make sure Android is up to date
Settings > System > Advanced > System update - Make sure Google Play is up to date (this can be more recent than the System update)
Settings > About phone > Android version > Google Play system update - Clean the scanner if really (visibly) dirty
- Delete all registered finger prints
Settings > Security > Fingerprint - Reboot the phone
- Set up at least one finger.
It can help to record the same finger more than once.
When registering the fingerprint, at each touch, leave the finger on the sensor for about 3 seconds until told to “Lift finger then touch sensor again”.
Once at least one finger has been registered, you can use it to unlock the phone except the first time after a reboot. Using the fingerprint is not an option listed under “Choose screen lock” which is managed separately and offers None, Swipe, Pattern, PIN or Password.