You may also want to give these guys a call - http://www.partsgroup.com/
I have a P0421 code today.It is for a toasted first CAT! Mine is CA car and has 2 CATs. It seems the first CAT in the CA cars last less than 60K. I had it replaced under warranty when I bought it with 55K miles on the clock. Now is has 114K.
Only Mazda has this CAT and is over $400!I am going to go w/o the first CAT by converting it to using 1 CAT only. Gary says he can help me on this. The moral of the story is to stay away from the 99-00 non-fed cars, unless you are ready to buy an OEM CAT every 50K miles or so!
Gabriel
You may also want to give these guys a call - http://www.partsgroup.com/