yo
If you wanna find the exaust leak hold your hand over the end of the exaust with the car idleing and you should hear it.
First off a word or warning on the O2 sensor. You will need a special socket to change it. I belive its a deep well 17mm with one third of a side cut out of it to allow the for the wire. As for your tools. Keep an eye out at flea markets, yard sales and such. Ebay is also a good place to look. Craftsman is a good starter tool. I have broken more than my share of Craftsman ratchets and split a few sockets. Since then I have become faithful to Snap-On. Another I feel is good quality at a cheap price is Stanley. Wal-Mart sells them. SK is pretty good too but costs more. Back to your problem, try running some Techtron fuel system cleaner thru the gas. If you change the cap and rotor, after you get the cap off put the car in 5th gear and roll it to get the screw to where you can get to it. This screw can be a bitch to get out so be careful and patient. After you replace any electrical component like the O2 sensor reset the ECU.