o2 sensor

Hi, I pun Buick lacrosse 2011 cx 2.4L. Where exactly is the O2 sensor bank 1 section 1 located? Is it the one after the engine at the top? Thanks for help.

My catalytic converter rusted out and I had to replace it. Now I get code po420. I didn't have a code before this replacement. How do I know which sensor is bad? 2007 Buick Lucerne

My front upstream O2 sensor that came with the car died, I think it was even factory. Came with the codes PO131 and PO171, and also worse MPG. With my somewhat fancy scan tool, I was able to see that both fuel trim levels were stuck on 1 number and never changed. I bought a Denso O2 sensor and...