I have had the same thing on the 99 Contour that we have. The check engine light came on and I pulled the codes almost immediately. It was a O2 sensor, so we replaced it. (Thanks Chris). And then it went away for a little while and then came back, then went away and came back, and then went away and hasn't been back since. It always came up with the SAME P0420 code. I have a used (51k miles) set of Catalytic converters off an SVT, but am not going to put them on until I am sure that they are the problem.
Next time it pops up, if it does, pull the code (Auto Zone etc) and remember the NUMBER they pull. Don't listen to the guy at AZ....
