So I have had the same code since I moved down to Texas almost a year ago. P0420 "catalyst deficiency". I have replaced my cat twice(under warranty) oxygen sensors.
Checked, cleaned, regaped plugs, checked injectors, every thing is firing, run cleaner through it.

At 2,500 RPM the temp before the cat is 340F and after is around 450F, So I know the cat is working

I just fixed the leak in the system so I know it's not a leak, this sh*t has me stumped.

Also I get like 20-22MPG highway which seems kinda low

Some one PLEASE give me some advice

I've come to the point where I have just removed the MIL light cause the bright orange is no fun to stare at.

EDIT: I pass inspection by resetting it and not driving more than 40 miles, Yes it is doing its job(Temp test proved this), but must not be to honda' ECU standards

1999 V6 Acord 180K miles

 

what kind of bicycle are we talking about here ??? Input all the info in your signature ,,,,

did you use Honda factory cats and o2 sensors?? If not,,those after market cats and o2s are not worth crap,,,,,,,,been there ,,done that ,( 2 different cars, warrantied cats 3 times ) ,and code came back after a few months , used Honda cat and o2s ,, fixed the issue ,, on 2 separate cars 98 and 99 accords , it has been now over 2 years ,code has not come back ,,
also make sure that there is no exh leak at cat mountings

 

There was a leak from broken flex pipe, but that was replaced. I have no idea who makes the o2 sensors, light is just so da** annoying
Cat is made by "Shultz" or something of that nature.

 

Idk why these cars are soo picky, but deserthonda is correct... If you want the cel to go away, replace the cat and o2's with factory oem honda genuine parts... May cost you a little extra, but it will save you from the bright orange light...

 

I'll save myself a $1000 and take the light out, cat dose what it's suppose to but not good enough for honda