0420/0430 codes are "common" on the 2.6 / 3.2 engines .. it's not a matter of "if" they occur .. more "when" they occur.
The "fault" - and IMHO it is NOT a "fault" as such - seems to be triggered by a "change" in the exaust gas composition that passes through the small pre-cat .. the sensors are either side of this and NOT the main cat.
I get the fault twice a year, almost without fail .... just after arriving in France, and just after returning from France ... ie, after refuelling with a "different" fuel.
Now there is no "proof" of this as such .. just my experience. I remove the codes with the GS500 and things are fine for the rest of the year.
What folks are saying... is you may have changed the fuel you put in ... another service station/supermarket/different fuel .. It's also possible that running the tank low has dragged some crap from the bottom of the tank through the engine, the burning of which has "upset" the sensors into thinking the cats are failing ...
by refueling with a "quality" fuel for a couple of tankfuls the light might well go out on its own as the system settles down.
If not, post up your location and there may be someone close who can use a code reader/resetter to do it for you..

HTH

edited for smelling pistakes ..
