Does the truck have an aftermarket exhaust on it? If it does, that could be the reason for the light. Especially if the catalytic converter has been cut off. This causes the check engine light to come on and stay on. It has happened to all my friends trucks that have cut their converters off. If this is the reason, you don't really have anything to worry about. There isn't any performance loss or anything like that. It could also have an exhaust leak. I'd definately test drive it and have it checked out before buying.