Pretty scary stuff.
If you take your car to a dealer and they tell you it could be a number of things, they are tring to rip you off. With the diagnostic equipment they have, they know everything your engine is doing and the ECU records errors for each device that can fail. In some extreme cases, multiple codes can come up, but it is usaully a ECU or electrical problem causing the multiple codes. If your dealer can't tell you what the problem is and if they can't guarantee a fix in the first try, never go back to that place again. They are taking you for a ride. Not a good ride like a rollor coaster, but a bad ride involving a rubber glove without any lube at all.
Also, read your warranty. Now days cars come with a 7 year 70K mile emissions warranty. I belive that is forced by the EPA in all 50 states now. So, if your cat needs replaced, it is at the cost of the dealer. That might even cover o2 sensors since that is a part of emissions. It covers EGR systems, exhaust systems, cats, cracked headers(any exhaust leak), everything. If you are denyed emission warranty, get all the information from the dealer, manager, and then contact the EPA and inform them. Many people get screwed on this kind of stuff. A Ford dealer tried to charge my poor folks for a new cat. I called the dealer, had a little chat with them, and they took care of everything.
02 sensors can fail as early as 60k, but they usually fail around 80K miles or longer. Pump gas should never cause a 02 sensor to fail unless you are buying gas from bob's greesy spoon fill and puke station.
Dealers make most profits from car service. It is a very ugly game and many many many people break the law and get away with it. You can't belive everything you hear, but dateline does a pretty good job collecting data. They've sent cars out to get repairs and most of the time they were charged for something the car didn't need. Other times the real problem wasn't even fixed. Simple stuff too, not sneaky crap that would be hard to find.
I used to take my car to shops for everything including oil changes. About 8 years ago after getting my car dammaged by a oil lube shop, I started changing my own oil, spark plugs, and all easy mantainance. Over the years I took on bigger task and have collected more tools. Now days I do almost all my own work and I hate having to take my car in for anything. I don't touch a car under warranty, but anything outside of warranty, I'll fix myself. About the only thing I can't do is change and balance my tires. I HATE having to go to the tire shop. I don't like anyone touching my car because I know that 80% of car techs are not very educated and many of them take short cuts. I know a few of them. They are not nice to your car, they don't use torque wrenches, and hell, they might not even bother reading up on how to do the job right.
So, what can you do? Start learning how to do your own work. For the cost of one expensive trip at the dealer, you could buy manuals for your car and a pretty darn nice tool collection. Stanly tools are pretty nice now days and they don't even cost that much. One day I broke a crafsman socket and sears was closed for the day. I bought a stanly set at wally word and finished the job. Sears is the only place I trust to truly warranty all hand tools forever (exluding torque wrenches). Local clubs and forums should be able to get you started.
As far as reading codes, you have to use the local parts store, or maybe borrow a club members code reader. I don't even know if you can buy a afordable Nissan code reader. I havn't been in the Nissan game for very long and I'm getting out. I have one for my other car and I know ford and GM ones are pretty cheap. A universal ODBII reader should be able to grab basic emission and universal codes, but auto manufactures are allowed to have a long laundry list of codes that only a dealer can grab. Now days, they can collect some pretty scary info you might not want them to know.