Yes, it is easy to replace. I have had to replace the upstream and downstream in my Contour. The upstream failed at about 50,000, then the downstream went at about 58,000 actually a few weeks ago.
The upstream will effect your gas mileage badly, downstream somewhat.
I didnt replace it myself so I cant help ya out to much but to replace upstream, but if you look at your car down by the radiator there it is. You need a O2 sensor socket maybe to pull it out, then follow the wire wherever it leads to unbolt it and then replace it.
Downstream the car must raised up to do.
Do you have any CEL codes?
