[spacedout]

Just because the codes is for a sensor doesn't necessarily mean the sensors bad, it could be as simple as it doing its job and pointing to something out of spec. With the boost sensor it could be an issue with the turbo(low boost) , that is all it would take to set that code. Do not throw parts at the problem, it would be better to take the car to a mechanic(preferably a chevy dealer) that is more familiar with the car.

 

[spacedout]

What indication is there of an aftermarket tune? Just because the car didn't have a muffler? I also doubt putting the muffler back on had anything to do with the CEL and was just happenstance it occurred sometime afterward. I see no reason at all to treat the OP CEL any different than an unmodified car, the code should point to the cause of the problem.