Need some input here: CEL came on yesterday after a 350 mile journey. It was running the COBB stage 1 tune (the only modification I have on is the complete mountune cold air intake kit). Checked out the error code on the accessport and got the P0420 error code (catalyst system below efficiency threshold, bank 1). Reflashed the car to stock tune in prep to taking to the dealer and now the CEL is gone. The flash port isn't detecting any error code. Drove it on the stock tune for a bit...CEL didn't come back on...reflashed it to the mountune tune and drove it more...yet no CEL..car seems to running fine...appropriate boost n all...not sure what to make of it?