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?
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