2001 Chevy Cavalier check engine light came on

Here is the history at 72000miles for 2001 cavalier:
put new cat converter on
replaced oxygen sensor near manifold
Sometimes the check engine light comes on and everytime I check it with my harbor freight OBDII tester, it says P00420, 01/02, so when I look that up in the book it says catalytic converter not optimal, bank 1

I just put in a new Bosch oxygen sensor in the manifold and after 50 miles it came on again. Does anyone have any trouble shooting ideas?

When new, I got 34 miles/gallon
Now I get 26 miles/gallon
The low MPG may be due to the new ethanol in the gas. When that started, my MPG went way down on my truck as well. Maybe 2001 technology at GM was not designed for ethanol in the gas at 15% of volume.
I have just filled up the car with a higher octane of gas hoping it will burn off any residue on the new oxygen sensor, should I try some fuel injector cleaner next?
Thanks, Rich