Hi! I (still) have a 2001 Prius which has been throwing P420 errors over the past few years. They seem to coincide with the ethanol fuel mix here in MD, I noticed a similar problem on my 2002 Sienna. Replacing the catalytic converter is an option, however was there a CPU/MCU upgrade for the 2001 Prius in order to handle the ethanol fuel, and any other options aside from swapping the cats? Car has about 210k miles, I rebuilt the pack with Gen 2 cells at 150k, recently replaced the plugs (why not), and I will probably keep the car forever.
No upgrade apparently however I did replace the 4 COP (coil on plug) units and the car came to green within a day. And just passed emissions. So maybe mediocre COP units can do it.
