Check Engine amber problam/question

I have a Land Rover Discovery 1999 seria 1, the check engine indicator illuminates for the last 2 days, the engine operats normally and the vehicle drives normally. I did check a flue filler cap if is fitted correcly, and the Check Engine light still illuminate.
Did anyone knows what's the next step to be check and what could be a problem.
Thank You,

the datve is 16 march 2006 my 97 disc check engine light came on on I95 at 4 in the morning three miles later it is sitting on the side of the road, it just shut down, might want to get it checked man,