Getting sick of seeing emission codes

Have a '98 Prizm with 143,000. Had two emissions codes PO441 (purge valve) and PO446 (evap canister), replaced both items. Turned off MIL, it stayed off for about a week. Today MIL came back on, only a new code PO420. It seems like I fix one thing in the emission system only for something else to come up.:bricks1: Will it ever stop? Or will it be vicious cycle. You know back in 2003, EPA sued Toyota for having a screwed up emission systems on '96-'98 Corollas and Prizms. If you are an original owner of a Corolla with the above mentioned years and have less than 150,000 miles. You can get your entire emission system problems (from gas cap to intake manifold) fixed for free by Toyota. Check out link, look underneath your hood on drivers side for the info. http://www.epa.gov/compliance/resources/cases/civil/caa/toyota.html
Scroll down to the 'list of vehicles PDF

Bad news for '96-'98 Prizm owners :swear:(Prizms and Corollas were on same assembly line), actually there were two lists. My Prizm was on the original list. But after Toyota complained to the EPA that it didn't trust GM to fix the problem correctly on the Prizms, EPA took the Prizms off the list. So now me and other '96-98 Prizms owners a stuck to fix their own emission problems. So far vapor canister purge valve, I installed myself $65; Evap canister ($285 + 50 to install- I don't have a lift). So back to my OBDII code PO 420, which are O2 sensors ($75 ea). The one in the front (easy access), back one (remove center console). Oh boy what FUN that's going to be!:thumbsup: Or it could be the catalylic converter ($ ?), I'm sure no cheap.