Call me skeptical, but I don't think it says "your cat is bad".
I'm guessing it's a P0420 which doesn't mean "your cat is bad" but rather it means "The ECU is receiving signals from your A/F and O2 sensors that indicate there is a problem with your emissions system".
It could be an exhaust leak.
It could be a vacuum leak.
It could be an injector.
It could be an A/F sensor.
It could be an O2 sensor.
Or it could be your cat.
Since you're under warranty, just take it to the dealer and have them do a drive cycle and diagnose it properly.