Last night I took my car on about a 3 hour round trip and just before I was pulling in at the mall my check engine light came on, well I drove the 2 minutes on to the mall and turned the car off and started it back and it was gone and hasn't came back since. Is this anything to worry about? Also I found in the ashtry of the car a little key that has gm and autozone on it, its made weird on the end, could this be the little tool for getting the cel to flash to read codes? only thing weird the car does/has done is idle slightly high at times and it cranks a few extra seconds before it fires up.

 

Our cars are OBDI. If the problem goes away, the check engine light will go away. It will be stored in the memory of the computer until you erase it. SO you can go back and see what it was. But if it goes off, the condition went away. No idea what the key is you are describing, i'd have to see a picture.