Engine Error Code P0420 and Transmission leak after CV half-shaft replacement

[Aveofest]

I have a check engine light on...and I went to Autozone to have a diagnostic check.
The code that came up was P0420 (Catalyst efficiency low-bank 1)

They suggested that the probable causes could be:
1. Cataytic convertor defective (failure possibly due to #2, 3 or 4)
2. Engine misfire or running condition
3. large engine leak
4. engine oil leakage into exhaust-valve guide seals, piston rings.

Well this all be said..the engine idles perfect and doesn't burn oil. The head gasket was replaced when I bought it.

So I was wondering what you all thought...hopefully I only need to replace a oxygen sensor and not the catalytic convertor.

Also on another note I recently had my CV joint pull out of my transmission do to a bolt falling out of my control arm....this being said, Big-O Tire had done a alignment on it a few weeks prior to it, so they took care of the repair costs, but now it is leaking transmission fluid...is there a gasket that needs to be replaced when the CV half shaft is installed?

Any help would be great!

[freddingo]

Sorta bringing back the dead here... but I cleared this code by tightening the gas cap.

[MetroMPG]

Wow - that's good news. Better than those other options!

myfakeevo cleared that code after tightening the gas cap, and it appeared to fix it: http://www.aveoforum.com/forum/f82/p...83/#post136301

and this site suggests that code could be gas cap related: OBD-II Trouble Code: P0442 Evaporative Emission Control System Leak Detected (small leak)

Also: http://answers.yahoo.com/question/in...4165453AAB7WTv