Is it a 1133 code??? All that is saying is that it once the car reaches closed loop mode that it didn't switch between rich and lean enough times in a given amount of time. I think GM cars is 90 seconds is how long it runs the test, figure it wants to see 60 switches but it saw 55 or 58 more than one time it will set the code. I dealt with it when I did headers and after a few times it stopped popping up.
If so, just clear it. It's a pretty common code anytime you have headers or aftermarket manifolds to the car.
