As I understand it,when this appears,it means you have an emission problem.
My gas cap was broken and I was running my truck w/o my gas cap on.
Would this cause the light to come on? Also,once it's on,will it go out on it's own? X

 

that would be the cause... its also very dangerous, as im sure you know. You will have to rest the code, but the light can come on for any reason, not just emissions