Fingerprint sensor works fine for me. Nethertheless the detection rate is much better with recently learned fingerprints - if you’re working a lot with your hands it seems to be a good idea to remove and add the fingerprints about once a month. This really saves time everytime you’re unlocking the phone.
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I have problems too and use the sensor in different apps.
I put on strip tesa on the sensor. Now it works great (recognition rate 95%)
Unconventional but cheap 
Maybe the sensor is too sensitive.
Have a try. Maybe it works on your phone too.
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Adhesive tape (Tesafilm TRANSPARANT) on the sensor still works after 2 weeks.
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I put on strip tesa on the sensor. Now it works great
Thank you for the advice 
I gave it a try for 6 weeks - with tesa/ahesive film, the fingerprint works reliably in all conditions, i.e. usually 1st try is a success, while after removing the film, around 50% of login tries fail completely, i.e. using several fingers several times until fingerprint is de-activated due to too many failed tries. Moreover, with film, I virtually never saw the message “sensor dirty” while in fact, it was clean - without film, this happened many times a day.
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For me too the fingerprint scanner works way better with adhesive tape on it. Which is very nice and and I’m glad about it.
(Well, as long I don’t think too much about the fact that a little piece of adhesive tape, that probably costs less than one cent improves the user experience of a device that I paid 450 Euro for by that much, that is.)
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For me, a tesa strip mitigates the issue a bit and it’s certainly better than without, but we’re talking unusable compared to barely usesable. Detection rate is still very low for me (I’d guess ~20% on average), even requiring a manual pin unlock about ever three unlock tries since the sensor gets disabled… the more humid it is is, the worse it gets, same if it’s too dry. Not to mention that the sensor is often triggered when the phone is in the pocket and gets disabled again. It’s even a step back compared to my previous phone which had a smaller sensor on the power button on the side, which wasn’t great either but still worked for 3 out of 4 scans. It’s really disappointing, that a manufacturer so keen to be sustainable uses a component so bad that you are basically forced to get a different phone when you are relying on this functionality. I hope there will be an upgrade for this sensor soon.
This was the single most helpful advice about the FF3 fingerprint sensor.
It takes time!
While my previous phones required only a light touch with the finger, even just scarping it, the Fairphone sensor works best if I press my finger firmly, for the better part of a second, always at the same angle, into the middle of the sensor.
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Just came here to say that I’m baffled but indeed putting a simple piece of transparent tape over the sensor makes a tremendous difference. Before the tape the sensor only ever worked for me in optimal conditions, which was almost never. I don’t know if I have an atypical amount of moisture on my fingers or something, but just the act of typing on my warm laptop makes my fingers “greasy” enough for the sensor to register as dirty. Same if I washed my hands in the last few minutes, unless I manage to get them extremely dry. Same if it’s cold outside - this has nothing to do with my hands, but if the sensor gets cold there tends to be a bit of condensation on it, which, too, renders it useless. Curiously I sometimes use my phone in the gym with quite a bit of chalk on my fingers, but as long as my hands are dry it’ll work.
I feel like the sensor is just super sensitive to any kind of moisture. How exactly the tape mitigates this is unclear to me, but since applying it I haven’t needed to type my password because the sensor got disabled for too many attempts. There’s still the occasional miss, but it now works in situations where previously I wouldn’t even bother trying and went for the password directly.
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Thank you for this tip! I’ve put tape few weeks ago and it works perfectly so far. After putting the tape I’ve cut it with craft knife to fit perfectly, so it’s not visible.
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Bravo, I put some tape on mine, it’s dramatically better, I haven’t had the ‘fingerprint sensor is dirty’ message since
The different experience are telling, telling what though I don’t know.
I’ve had the FP3+ for 10 months and use the fingerprint reader regularly and never had a problem, no notification of dirt etc.
I work outside on the land and regularly have soil or maybe grease and paint on my sweaty hands but I usually wash them before using the phone.
I use a case which means my finger has to be orientated more carefully so maybe that’s the secret ? Oh! and with all the washing nice and soft 
Hi, I have had lot of problems with the fingerprint sensor as well and I would like to share the solution I adopted. My skin hydration changes a lot as seasons change. When I understood that this variable was affecting the sensor, I implemented this workaround: I stored a new fingerprint of the same finger, each time the previous one stopped working (I added a new one without erasing the previous ones). Now I have 4 fingerprints stored and the phone is working perfectly fine since the beginning of the year.
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Just adding this though I’ve given it a ‘like’ it’s a very useful tip and needs to be noticed.
Thanks for efforts and success.
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I also only have bad experience with the fingerprint sensor. It used to work when the phone was new with a rate of about 30% and 1% after climbing as if my fingerprints would change. It’s hard for a climber to keep their hands dry as the thin skin on the fingertips perspire more easily. Now after 1 year I get the message that the sensor is dirty 90% of the tries. Even when I cleaned it and my fingers are dry. I can not even set up new prints in less then 30 Minutes. So it is absolutely useless and a big disappointment.
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Did you already try the tip from @Alex10
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It really does work way better now with adhesive tape! Let’s see if it still works after climbing
Edit: I had to add a second fingerprint of the same finger but it seems work more than 50% which is fine. I hope I don’t have to switch the sticky tape too often. Thanks!
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After 3 days I keep getting the “sensor is dirty message” again constantly. I switched the tape several times. Maybe due to the slightly warmer weather? I go back to being disappointed.
I’ve been meaning to post on this thread for a while.
When I first received my FP3 a year ago the sensor seemed to “forget” my prints after a day or two, and even when recording the same finger more than once, so I just gave up trying.
Then sometime back in the spring of this year, maybe after an update, I tried again, and this time it’s been working perfectly (except when I’ve just washed my hands, I don’t even bother to try). I’ve used no tape or any other trick, apart from multiple records, and it works perfectly. 
@xinran : since you’re a climber, I fear that the wear on the skin of your finger tips probably makes the patterns change too much for recognition. It would be interesting to hear from other climbers, or people with professions or hobbies that wear the skin a lot (carpentry, gardening, archaeologists …?)
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I manage 40 acres, grow most of food, work chopping wood carpentry, digging, washing clothes by hand etc. for 47 years or more and i have no problem
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I climb too and have not had a consistently good experience with the sensor, either with the tape or using more than one fingerprint. The fixes mentioned here seem to only work for a week or so and then get gradually worse over time.