I have a mazda protege 5 2002 and have been trying to get it inspected. It shows the code 0421. The car has 128+ miles and has never had a tune up so decided to do that this weekend. I want to replace all that I can in hopes it isnt the cat (which im going to figure it is) . It hasnt had a tune up so I figured now would be a good time. It has never ran bad and just recently cel came on. I can do the minimal things but looking online I see that there are people stating that there are more than 2 02 sensors. Is this true and where can I find the other 2? Please help.