My vehicle is a 1999 Pontiac Grand Prix and it keeps overheating.?

have your mechanic check the sensor that controls the operation of your electric radiator fan. your symptoms are typical for sensor malfunction. this problem is often indicated by your engine light being on. You can take your vehicle to several different parts suppliers and they will run a test with a scanner that can tell the reason for the engine light being. Autozone,Advance Auto, and Oreily are jest a few that will perform this test at no charge to you. There may be others in your area as well.