With the alternator being replaced it is most likely just a connector not reconnected. Fortunately Bank 2 is at the "Front" of the engine.
Make sure that connector is on and secure. If it is and the fuse is ok, then disconnect it again, and check the resistance. It is most likely open. A replacement sensor 226931AA0A is about $200 from nissan but you may be able to find it cheaper.

Re your question of how serious? Well it works with the ecm to adjust the fuel air mixture. It can lead to rough running (especially when cold-ish) and eventually, even cat failure if it has gone on long and bad enough but your car will really run poorly before that occurs (my crystal ball not anyone elses!)

I would take care of it as soon as you can, hopefully, just the connector loose after the alternator surgery. (Yeah, I know the $11-$1200 hurt!)

Let us know what you find and good luck

Lt

 

 

I am envious. Most of us had to pay 4 digit as the AC has to come out (which means service too) and the alternator alone was 300 or so! Have that mechanic fix your connection and/or replace the sensor. You may get by for $25!!!