Some time ago in another post I did about my o2, someone mentioned that my fuel mixture may be bad because maybe the bottom sensor's on the way out as well (I changed the top one and get worse mileage then before I changed it). It seems fairly new to me....anyway, the question is.....are the top and bottom sensor the same? Or do I have to look for something specific? When the time comes (or if the time has come), then I'd wanna be sure that I know what I'm looking for.