first of all the o2 sensor measures oxegyen in the exhaust. It will read the burnt fuel mixture and if there is high levels of o2 then it will change the fuel mixtrue accordingly. check the voltage that the o2 sensor is creating because that is one of the only sensors that will make voltage instead of getting a refrence from the ecu. if the voltage is between .5 and 1 vac then it is rich there is too much o2 and the burn is rich, if there is between 0 and .5 vac then there it not enough o2. the votage will fluctuate so expect that and do it when its warm operating temp. If the cat is no good it will not pass no matter how many tune ups you do the cat is designed to get hot enough to burn the excess that does not get burned in combustion, fuel will have nothing to do with it. The sunoco that someone suggested is one of the worst fuels to buy it is a marketing ploy that works. they get their numbers chemically instead of naturally, it says right on the pumps, up to 10% alcohol. no good. Did you know that a good race fuel has like 75 to 80 octane?? replace the cat and it will help make it pass. Make sure you guys know before posting people really take your advice dont want to steer anyone wrong