Can anyone tell me which part I need to replace? I'm guessing the passenger side o2 sensor before the converter?

 

It is probably an O2 sensor, but have Nissan check this before you go ahead with replacement.

 

I'd love to but they want 115 just to run a diag and won't accept the self diag built into the Z. They wanted 115 to diag which airbag is bad, then charge me for everything and its had the problem every visit since 2008. I bought it new from them in 2005. All they ever do is "reset stuff". I'm ranting my apologies. If you had to guess though?