Never ignore check engine lights even if everything is seemingly ok. It could be a lot of different things, some harmless but some not so much. If it's something with a sensor or other intake part, it could also be taking away your gas mileage.

Pro tip: As the others stated, they make good obd-2 readers for cheap and you can also go to many auto parts stores and they'll use theirs to give you the code print out for free.