2000 Ford Excursion Service Engine Light

I have a 2000 Excursion with 107000 miles and several months ago the service engine light came on. I replaced the fuel cap and replaced a fuse. I then disconnected the battery for several minutes. The light went off and then a few weeks ago it came back on. I have checked the fuse and it has not blown. When it was on before I found out the terminal does not work so I cannot find out the code it is sending out. Do you have anything I can try? The car appears to be running fine. I have been wondering if the 4 wheel drive is not working. Could that be possible? How could I check to be sure if the front wheels are working correctly when the 4 wheel drive is on?