There have been a number of threads with this exact condition over the last month. Either scroll back a couple of pages or perform a SEARCH looking for "P0420", P0430", or just look at the titles.
Everything you need has been discussed.
Steve
98 Contour SE Sport 2.5 Duratec ATX The wifey's car
89 Taurus SHO - 246K miles
94 SHO ATX - 190K
1997 F-150 5.4L ATX - The Workhorse 150K.
ANY THREAD WITH "OMG" or "WTF" ETC IN THE TITLE WILL BE IGNORED!
