My o2 sensor is bad so I want to change every single one cause if they all the same age and 1 is going bad then the rest will follow. But here is the problem, I've gone to 3 Autozone and 1 side shop mechanic and it seems no one has a strong solid answer. 1st AutoZone told me (3) 1 upstream 2 downstream. The 2nd AutoZone told me (4), 2-up and 2-down. The third AutoZone says (3) 1-upstream 2-downstream. The mechanic says (4) 2-upstream and 2-downstream. I have not made the purchase yet because I want to make sure I have all that is needed before I take to service. Does anyone have the absolute, truly correct answer? I would really appreciate it. BTW One AutoZone was located in Baltimore and the other two were in NY.

Thank you.