1999 Mercury Cougar

P0420= catalyst efficiency, Bank 1, you need a converter replaced, and the upstream 02 is not switching, so you either need that or the wires to it checked, tho I would go with the sensor with this code...Bank 1 is the Passenger side

Thank U Merlin. I just change that oxygen sensor about 3-4 weeks ago. I assume that the wire that goes to it is faulty then. How does the wire get replaced and is it costly and difficult? The catalyic converter for this car is it difficult to replace?

The other reason you might not be switching enough, are you using the recommended fuel grade? If the converter is clogged, replace it before you start testing th 02....I use a sawzall and cut the old one out, then install the new one, it's best to weld them in, but you can try clamping and using muffler sealer to pack any leaks. As for the location, it will be tight!