Check engine light
 

Okay... I replaced the plugs and coils and the radio headunit in my fiance's 01 Golf 1.8T yesterday. Today her CEL comes on. Well autozone says the Cat is going bad due to blah blah blah...well the car runs FINE. Anyways..I'm new to turbo's and if I have to replace the Cat, what do I get? Can someone say wether or not a downpipe is an option?