Check Engine Light

I have a 1998 Pontiac Transport Van and it runs fine but I have noticed that if I drive above 110km for about 10 minutes the check engine light will come on. If I shut the van off and leave it over night the light is out and never comes on again until I drive over 110 again. It has come on occasionally when the gas station fails to properly secure the gas cap but that's all.

If you have no other symptoms that give some kind of a hint, then pulling the codes it the way to go . AUtozone will pull them for free.

Thanks my brother works in a GM dealership so I'll let them have a go at it.