Hi guys.

I was recently riding and my buddy was behind me on his motorcycle and when I got on it he said I emitted some black smoke. I know I need to replace both of my O2 sensors (Were bad when I bought the car)

I was curious if this is just the Primary (front) one that would be doing this? Not reading the proper fuel/air mixture?

As I understand it the rear one is just to check if the cat is working or something?

Thanks!

 

you are correct. primary 02 sensor helps control fuel trims, secondary sensor only check cat efficiency.