I am not sure what type of sensor the post cat one is, but what you describe may well be normal.
Pre cat sensors will typically be switching rich / lean but you expect the post cat sensor to be sitting at a fairly steady level unless there is a problem with the catalytic converter. If you blip the throttle then you should see the post cat sensor jump a bit (on a scope, but perhaps not on a meter).
Scan tool codes do need to be treated with care as what they mean can vary a bit - the out of range code you are getting can mean a couple of different things. It may be that the ECU is picking up a signal that is out of the allowed range for the sensor (perhaps 0V or 12V if there is a wiring fault somewhere) or it may mean that the sensor is working OK, but the catalytic converter is on its way out and under load the post cat sensor is switching rather than giving a steady signal.
Perhaps someone here with better knowledge of the Alfa ECU can clarify a bit further?