Trouble shooting cougar

I was driving the other day and my check engine light came on, I took it to the shop and had the code done, it was po420, does this mean my (cat) is no good, cause I just replaced the o2 sensor right after Christmas:banghead:

What year is your car? What were told the code is? The English translation that goes with the number. Was the problem O2 sensor before or after the catalytic converter?

Something to keep in mind
New parts doesn't always mean good parts. There could be a problem with an electrical connection.

contour.org is down right now, but if memory serves that code does mean the cat is bad.

It's a 1999, and I have replaced the top 2 sensors, cause they said the bottom two were good.