Is it the check engine light? Take it to Autozone and have them pull the codes. When your light is on, it sends codes out and each code stands for something different. That will help you diagnose the problem.Also, any reasonable seller should let you take the car to a mechanic and have him look over it if you want to do that. If he doesn't agree to this, knowing the maintenance light is on, he might have something to hide. So just watch out!