AoA sensors are a probe or fin on the side
Looks something like these.
The fin type is basically a fin on a potentiometer* (like a household rotary dimmer), the probe type measures differential pressure. I understand that the 737MAX has the fin type (as do most modern aircraft)
Any sensor could fail in flight - a plastic bag or helium balloon could wrap around it, a bird could hit it or crap on it, it could ice up or stick for other reasons, it might be fitted wrong or burn out young etc.
So if the sensor is really important, you have at least three, of at least two different designs and placed in different locations so that it's really unlikely that two would fail on the same flight (hit by the same object, ice up together etc)
With three sensors, you can tell which one is broken - the other two agree - and thus fly to an airport where they can fix it before you fly again.
If two break on the same flight, it's even less likely that they'd break in the same way at the same moment, so you can tell that at least two are broken - but you don't know which to trust and should ignore them all.
With only two sensors, if one is broken then you cannot tell which one is right, so you should ignore both if they disagree.
With only one sensor, if it breaks you simply don't know.
*Or other type of absolute encoder
Biting the hand that feeds IT
