2003 Chevy Impala P0420 2003 Chevy Impala

2003 Chevy Impala 6 cyl Automatic 90,000 miles

I would appreciate any help or advice. The Check Engine light just came on in my Impala this week & I had it scanned at Autozone and the problem came up as a P0420 code. Just a little background.I had my Catalytic Converter replaced (& both O2 sensors) back in July 2008 at a local AutoTire resulting from severe acceleration problems. This fixed the problem at the time & have had no problems since. In December of last year (2009) my Check Engine light came on & I ended up having to have the Thermostat replaced on Jan 9th of this year (approx 1 month ago) also at Autotire. Everything was fine until the Check Engine light came back on this week & Autozone's scan is now coming up with the P0420 code. Any ideas? I am having no accelerating issues at all like back in 2008. Could the thermostat replacement have caused the P0420 issue or be related? Seems like I read the possibility of this on another thread/website(?) But must admit I did not understand the explanation as I'm not auto savvy by any means. Thx