check engine light

i got a 89 b2200 carbed, and the check engine light comes on usually only on the freeway when doing a constint speed and if i let off the gas for a few seconds it turns off. what couldd cause this. and how can i read the check engine light, is there a code reader of some sort??? thanx for any help.

What is your temp guage doing when lite comes on? Truck might be o'heating and when you slow down the temp goes down. Is your fan working ok on engine. These little trucks are notorious for having fan problems.If you hear a loud roaring sound a few seconds after you crank up, fan is probably ok. If not it may need to be replaced. If so I would replace with a good electric fan. More efficient and less drag on engine. BJ