O2 sensors?

Hello,

Typically the oxygen sensors are categorized as upstream and downstream.
The upstream one located before the catalytic converter in the exhaust manifold and the downstream one located after the catalytic converter.

As far as which to replace, it depends on the one that you are having the issue with. Typically when you scan the vehicle for oxygen sensor code it will tell you which one might be at fault.

Replacing an oxygen sensor is fairly straight forward process. You will need to use a oxygen sensor socket to make the task easier.

Here are some helpful articles:
https://www.2carpros.com/articles/how-an-oxygen-sensor-works
https://www.2carpros.com/articles/how-to-replace-an-oxygen-sensor

I have attached diagrams for your reference.

Please let me know of any questions.
Thank you.