2002 Dodge Neon

Engine Mechanical problem
2002 Dodge Neon 4 cyl Two Wheel Drive Automatic 25965 miles

I have a 2002 Dodge Neon SXT which has a modified Magnaflow Performance Exhaust. Recently the fuel pump was replaced, battery, but my check engine light won't stay off. Goodyear ran a diagnostic on the car but couldn't test drive it because I didn't have tags on the car but now that I do, they're telling me that they have to charge me for another diagnostic. I was able to get info on how to get the code myself which I have and it's now showing code " p0420" & code " p1684" & I was able to research that it may be a catalytic issue & the p1684 is due to the battery being disconnected w/ in the last 50 starts. What should be the next steps to take?
January 7, 2010.

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