Check engine light

I have 4 year old accent and for last couple days check engine sign comes on while i'm driving.
Wondering ,what that might be??

if its 4 years old, its 2001? any vehicle build for the US market after 1995 has a check engine light to signify an issue in the emssions control system...first thing, and this sounds silly, make sure that your gas cap is tight.

Thanx .. It was O2 sensor

I get worried when my check engine light goes out! 1992 Scoupe

Why it's a good thing when the light is out.