There is no clear cut diagnosis when the light comes on. You can't just point to a component and say that is the problem. The first step is to have the codes read from the system. You will need a class2 scanner to get the codes a regular generic code reader will not get all the codes. Once you have the code it will lead you to a testing procedure. There are many expensive parts involved in the system so I recommend taking this to a chevy dealership to avoid the guessing games that untrained shops will play.