I just purchased a used 2003 Pontiac Grand Prix and after driving for bout 20min

Here at Security Auto and Truck we always recommend having a reputable repair shop look over any car before purchase. The check engine light can come on for more than 2400 reasons. Auto Zone will get you the codes but will not tell you what the problem is. Just because there are codes does not necessarily mean that that is the problem. Further diagnosis needs to be done. I agree with the reply above. 1st check the oil level. If that is OK then you need to have the oil pressure checked. Question for you, is the engine making noise when the oil light comes on? If it does not then you could have a faulty sensor. We would love to look at it for you. Give us a call 443-575-6641.

sounds like you bought a car without test driving it.if you are saying those lights are coming on after 20 minutes of driving.whoever sold you the car probably had problems with it and either cleared the lights with a code reader or disconnected the battery,hoping the service engine soon light would stay off just long enough to get rid of it! it's TAX time! FIRST,check the oil level,see if it's full,then take the car to Autozone,they will read the codes for free and print them out for you. post the codes here..who/where did you buy the car from?