New to Cruze talk have a few questions about warranty numbers and claims

So I have a 2014 Chevy Cruze. First had a cel code p11d7 I beleive about the nox sensor butbthat went away now I have the def quality poor message so I contacted Chevy they say they only have one recall or warranty for the def tank but while I was researching about the Cruze I found a few more topics and information and the 1st upstream nox sensor I beleive and seen people have had that replaced and ECU reflashed, wondering if anyone has any other I formation about this because the dealer says it’s my nox sensor #2 and has to be replaced stated the sensor is 500 and they have to make sure everything is working right, I’m trying to avoid this 900 dollar bill if possible