check engine light

I just got back from having my car serviced because the check engine light came on. At first I thought it was just cause the gas cap wasn't tight enough because it went off when I filled up my tank but then it came back on again a day later. So I was hoping it wasn't a major problem because I'm a broke college student and can't afford anything right now. I'm sitting in the waiting room for about an hour and the guy comes in and says you're all done. So I ask what the problem was and he says there was a huge spider web covering up the vent and that's why the light came on. Never would I have guessed it would have been a cobweb causing the problem. They were laughing but said surprisingly it is getting more common during this time of year.