code po420

i have a 1998 pontiac grand am with a 2.4 lit. motor. i scanned it the other day and the code came up as po420. can anyone pls tell me wht it is and possibly how to fix or correct it. i also have anothe ? . i have found one oxygen sensor on the exhaust manifoldit has a purple single wire coming out of it. now my ? is i was told there are 2 o2 sensors when is the other one at and is it the up stream or down stream sensor . any help would be greatly apreciated.

The H02 sensor near the exhaust manifold is upstream , the one after the catalytic converter is down stream. The down stream HO2 sensor signaled the ECU that your catalytic converter is bad.