Check Engine Code P0421 on 2003 Hyundai Santa Fe

What does code P0421 indicate that I should have repaired?

Asked by for the 2003 Hyundai Santa Fe
A code will never point to one item as needing replacement, it takes a diagnosis of the related systems to determine the fault. Here are some things to check for that code:
A code P0421 may mean that one or more of the following has happened:
* The catalytic converter is no longer functioning properly
* An oxygen sensor is not reading (functioning) properly
* A spark plug is fouled up

1 more answer , 1 more comment
read it somewhere in the manual.timing belt i think.
why is it so important to change the timing belt?????