1991 Chevy Truck Service Engine Light

This truck has had better days but I just bought it. The other day the service engine light came on after about 4 miles of driving. I turned the key off and then restarted the truck and the light was of but came back on after about 4 miles of driving again. This happens everytime. Since I purchased the truck I have done the following: Changed the oil and filter, changed the transmission fluid and filter, and changed the spark plugs. Any ideas what could be causing this to happen? The truck still runs fine even when the light comes on. I checked the gas cap and that doesn't seem to be the problem so I am really confused as to what to check next