Haven't had this 02 Qx4 but for two weeks, been scrubbing away at it trying to get rid of the accumulated stupid. Had the 420/320 codes from day one, right rear cat was rattling so replaced that, still had both codes. I bought CARB cats which haven't even arrived yet, but a couple days ago I filled it up with ethanol free gas and drove the pee out of it... codes went away haven't come back (yet), and none pending either. Guess I'll keep the cats but thought that was pretty interesting that good gas and the old Italian tune up made them go away, for now. Soon as/if they come back i'll post a follow up