O2 Sensor
 

I have a new catalytic converter on order and am just wondering how many of the O2 sensors I need to replace along with the CAT to get peak performance and also to make sure that I don't damage the new CAT.

After doing some research I found that usually it is the downstream sensor that goes faulty and needs replacing, but I also read that it is good to change all of them.

Is it really necessary to replace ALL of them, I only ask cause they are around 50 bucks a pop!

Any advice would be greatly appreciatted.

I wouldn't replace any of them unless/until you get a code.

That being said, most manufacturers recommend changing all of them at 100,000 or 50,000 miles or 75,000 miles or take your pick to ensure optimum fuel milage.