2002 Saturn SL2 Service Engine Soon - Evaporative System -

Engine Performance problem
2002 Saturn SL2 74000 miles

I have a 2002 Saturn SL2. The service engine soon light came on in October. I took it into Sears because it was slightly overdue for an oil change and they told me it might just be the oil change and/or a need for some fluid changes, etc. So I paid them to do all that. The light went away for about a month and then came back. I was then advised to take it to a mechanic who could run the codes, etc. So I did. The mechanic told me that the Sears person was an idiot and it never could have been the oil change or anything to do with my fluids. He told me he could figure it out. He charged me to diagnose the problem. Of course. And then said the code was for the evaporative system? And that the only thing he could find wrong with that system in my car was a cracked gas cap. He charged me $20 on top of the diagnosis fee to fix it, said he'd checked it three times to make sure he'd solved the problem and said I was good to go. So basically, I ended up paying more than $100 for him to put on a new gas cap. I was fine with that but. It's been a couple weeks and the light is back on. What now? Any ideas? Are there some simple fixits I can try on my own before taking it back to the mechanic? I don't want to pay another diagnosis fee if it's something like the gas cap again. Although I'm assuming that's not the problem because he just replaced it a couple weeks ago. Any ideas what it could be? The mechanic assured me that the gas cap thing had fixed the problem. But I've got a service engine light. And I simply can't afford to take the car in for a diagnosis every other week. I'd like to learn how to at least check for the simple stuff on my own? Help? And thanks.