1999 Honda Odyseesy code P0420

My 1999 Honday Odyssey keep getting the P0420 engine warning light
code despite the fact I changed both of the O2 sensors and the entire
catalytic converter.

I called Honda and they say really the last step is to re-flash the
computer. That sounds drastic to me. Before I do that I want to ask
the experts here: what else can cause this?