Ever since I had a misfire on two cylinders -- about 2 years ago -- I've had a periodic 420 and 430 code. I figure some unburnt gas 'clogged' the precats to some degree. My solution -- after the stealership replaced all the HT wires -- was to put a can of Berryman's B12 into the gas tank and just dirve the car. After I had done this 3-4 times (i.e. over a distance of approx 1,000 miles.) -- the CEL STOPPED coming on. :-)
I haven't had the CEL come on since -- and now, as a matter of routine, every 3 tankfills (or so) I put a can of BM in the gastaknk -- and, oh yeah, once a year I clean out the UIM (upper intake manifold) area and also give my K&N filter a really good cleaning.
Just my $0.02
G.