Back in 2011, my P0420 code, cat efficiency issues, came on after a 17 mile downhill run with engine braking. I found a Youtube video by radio personality Scott Kilmer which said to put lacquer thinner in the gas tank.
I did a lot of Googling. Everyone who tried it said it worked, without damage to the motor, except the owner of a much older Honda who said it did induce damage to the fuel system.
Everyone who had never tried it said it would damage the fuel system.
In my case, after some weeks the code went away. Not long ago it came back. I reset it, and drove to the border and back again, some thousands of miles, with no further code. The output sensor runs around 0.7 which does seem a bit high.
I found a Youtube video by a person who said he tried lacquer thinner and it does go out. But, after not long it comes back on.
Just now on Sienna Chat, a man said he has the same results with “Chevron Fuel Injector Cleaner with Techron (the latter is reportedly important based on other posters)”. Goes out long enough to pass verification, then comes back on. He says he has done it three years just to get past verification.
That would probably be less scary since at least Fuel Injector Cleaner is intended to be used in car fuel systems. And, the use of an automotive designed cleaner instead of lacquer thinner is the only reason I am bringing up this dead horse.
In my case, I will eventually install new output sensor. And, if I had the problem the man had, where it comes back on, I would probably replace the cat after while, if sensors did not fix it. But, that is my viewpoint.
Others don’t care since the light comes on at 95% efficiency.