My car developed a CEL Monday immediately after start up. I went to Chevy today and they diagnosed it. Apparently there are 2 sensors of some kind that monitor the exhaust or are on the exhaust. The sensors are what I was told by the Tech as a part that is "RESTRICTED". Meaning the part is not an off the shelf part like an O2 or NOX sensor and can only be obtained directly from the GM factory when the specific issue requires this to be replaced. There are 2 of these sensors and the only way to tell which one is bad is testing when the car is cold...not at normal operating temp. They are going to look at the car again in the morning when the car is cold to determine which one it is. They can't simply order both apparently and be done with it. GM won't allow this. Does anyone know WTF is going on here?