check engine code p0420

2009 prius tech pkg just turned 160k km / 100k miles and started to get check engine light for first time ever. obd2 scanguage says error code p0420 translates to catalytic converter out of range. oh oh, from reading this excellent forum I found that this is a recurring problem that will probably not be covered under warranty in Canada and will require the replacement of the cat for around $2000. I don't believe that is the best solution!
This is the same problem that I have also experienced on my previous 2003 Mitsubishi Lancer same thing after 150000kms p0420, dealer says cat was previously replaced under warranty recall only one per customer pay $2300. My response was "I will live with the light" One of my brothers kids got the Lancer once I got the Prius, and he said he fixed the cat error. Wth how? He said he used the Seafoam treatment. BS. I thought, I had the code for 3 years! and no $10 snake oil treatment can fix a manufacture defective cat problem. But the light was off and I know he din't replace the cat.
So to shorten the story and get to the solution. I tried the Seafoam fuel treatment just pouring a full can into a half tank of fuel. after 100km the light went off! Holy Crap I can't believe it. I am not easily duped or impressed, so I waited until after 1000km and 2 fills the light is still off.
Seafoam works!
I think I owe my brother more respect. Thanks buddy you saved me 2 grand.
If anyone else on this forum has the same experience please post.
I was a non believer too. I don't endorse any products ever, but if it saves other people from needlessly replacing cats it is all good. I hope it works for you too.