Frustrated with my celica!?

I've had my 2000 celica for a little over a year now. Over the summer the check engine light came on, so I took it to a local shop. They did a smoke test on the car and found I had a leak through my gas cap. It was an aftermarket one and so we replaced it with a toyota cap. Fast forward to a couple months ago, my check engine light is on again. It's pulling the 0440 code. So I took it back to the shop, and once again they say, "oh you need a new gas cap" luckily it was all under warranty and didn't cost me a dime, but 2 days later my CEL came on again. I'm really frustrated with my car and want to know what could possibly be wrong BESIDES having an issue with the gas cap. Because clearly the gas cap isn't the problem. I'm dropping my car off at the shop tomorrow... But unfortunately I know NOTHING about cars. HELP!