Cadillac Repair: SES on, smog test failed, seville sts, smog test

Question
Hi there,

My cadillac 2002 Seville STS ( 120k) has Service engine soon light on & recently it failed emission test badly as Gross polluter. The OBDII code is P0420

What should I do.

Thanks a lot.

Answer
Does the vehicle have the original catalytic converter(s) on the vehicle ?.

Are any of them damaged ?.

Do any of them get red hot after running the vehicle and getting out and looking under the vehicle immediately after stopping the vehicle WITHOUT shutting OFF the engine ?.

Do you have any other codes ?.