all the typing you did was very generous of you.

1. you answered and confirmed that i do have only one CAT and two sensors.

2. you did a whole lot of explaining here of the basics that i already know but may be will help other readers. what i got from "3" it is that i don't need to spend around $100 just to know that i do need to replace CAT and O2 sensors, but i will need to go ahead and replace the three parts and then go do the advance diagnosis if (God forbid) there was still code or problems. So, now i will need to research the most economical way. Is this an urgent job, from your experience? is there any engine effect for not replacing the three parts soon.
Do you have any estimate for labor? if direct fit or universal?

thanks much