Check engine light goes on- check engine light goes off.
 

I got a 99 2.5l boxster and the check engine light came on last week but it turned off today. WTF? the car fixed itself?!?!?

intermittent issues will cause that. For example a loose gas cap that was later tighten after fill up. Or a weak O2 sensor

You will be pi$$ing in the wind without a scan of the codes.

premium fuel ? I learned the hard way, thought mid-grade was OK...............NOT.

Mine came from the temp sensor starting to fail, low voltage, light went on for about a week then went off. Car needed smogged anyway and that was the only error code.

Yea, I guess I really just wanna know if it's something to worry about.
I'm looking to get one of those readers so I don't have to pay the shop for nothing.

Just go to your local Advanced Auto Parts (I'm pretty sure Autozone has it too) they have the scanner and will check the codes and tell you what the problem is for FREE. My engine light came on when I first got my box, and it was just an oxygen sensor that was failing. Pretty easy to change out yourself if thats the case.