It's almost that time of year to smog the car and I brought it in for a pre-test just to be sure. It failed and I went and replaced the catalytic converter. I brought it back for another pre-test and it failed but the emissions improved. I then changed the oxygen sensor and brought it back but it failed again and the emissions got worse. The problem area is in the NO section. The car has a had a recent tune up (spark plugs, wires, cap & rotor) and a fresh oil change but still performs poorly during the smog tests. I always run the car up and down the freeway for about 30 min. Before bringing it to get tested and I always use 91 octane fuel. I've run out of ideas and I really want to keep this car but I need it to pass if I'm to hang onto it. Can you guys pinpoint my problem? Thanks.