I have a 2000 Mercury Mystique 6cyl. About a two months ago I had the P0420 Code come up, went and had the Cat changed on it. About two weeks later the same code came up. Turned the check engine light off with the OBD2 scanner.And cleaned the wires going to the 02 sensors. And now yesterday 6/30/06 the check engine light came back on with the same code. I notice when the MIL comes on the gas mileage drops down. I mean way down, I get about 100 miles to a TANK when it comes on. Anyone have an idea of what is going on. The car has 118k on it. It runs great, will get up and go with the best of them. Thanks for all your help!