My CEL recently came on and it was pointing at the CAT. I cleared the code and it came back on in a couple of days. Well the other day it went off on it's own but has since came back on. After some research I think the quickest and easiest thing to check first is the O2 sensor. I jumped on partsamerica.com and they have about 8 O2 sensors for my 98 4cyl and they range from $70 to $300 but none of them really say what position they are for. I need to get the one after the CAT but don't know which one to choose. Is there a better place to buy them and is there something specific I need to look for as far as the location on the car is concerned?

Thanks in advance.