2003 Ford Expedition

First thing you need to do is have the codes read, Post the results with the code number(s). Even if the service engine light is not on, there should be freeze frame data stored.
If it won't make it, buy a cheap scanner, they can be had for 30-40 dollars, a good investment, since they cover all cars sold in the USA from 96 on.
If the light is blinking, it means possible damage to the converter. Service it right away!

If you need more help, post the codes and we can dig deeper!