Check Engine Light...

I have a 2001 A4, 1.8T. Car is stock, and maintenance is performed on time. About a week ago, my check engine light came on. I went to my local parts store to check the code, and it was System Too Lean. I don't remember if it said bank1 or bank2. The guy at the store recommended some fuel injector cleaner, so I tried that. I also thought it might have been some bad gas since it came on right after I left the gas station. I ran the car to almost empty and tried another gas. Same thing. The car seems to have lost a little power as well. A few days later, the light went off, and everything was fine. Then it came back on, and I had trouble starting the car. It sounded like a lawn mower. Now the car won't start at all. Battery and alternator seem fine. Any tips on what this could be? o2 sensor? vacuum leak? MAF? Thanks!