First let me explain I have searched high and low and by no means am I a noob.

The vehicle is a 2000 civic ex coupe

I am getting an intermittent P0420 code (reset the code and wont get one for month or so)

I have replaced the Cat, Primary and Secondary O2 sensors, exhaust gaskets, vaccumm leaks, Looked into the Honda TSB having to do with the distributer crimp connectors causing this code, and even inspected all the wiring from both O2 sensors for shorts etc...
New plugs, wires, dist cap, rotor, fuel filter

What else can be causing this code to come up

This code has been problematic for over 3 years. It has came up on the stock engine and now with the boosted engine and has not gotten worse or more frequent which leads me to believe it is not oil blowby or fuel related

Any info would be great.... Just wanna get rid of this code so I dont have to keep my code scanner with me all the time to check what the code is for...

 

Find out what is happening when the code pops up. Does it come on during vtec, idle, etc.

 

Usually after cruising for about a half hour or so at a constant speed such as highway driving. Vehicle passes smog with flying colors as long as I reset the code before I go to inspection. I'm thinking it has to do with something electrical

 

does the car have any type of egr system i dont think the sohc does? Also did you replace the cat with a factory cat? i work on vw's and they are extremely picky about what kinda cat converter you use. But you may want to check the fuel pressures and see if you have an injector that may be sticking alittle. i had a jdm b16 in my 2000 si and i kept getting a p0420 fault for the cat and i had obd1 injectors in it. i never thought that little bit of extra fuel woulda made a diff to the ecu.

 

The fuel system is a stock setup... Injectors are clean and are working properly... You say you were having problems with obd1 injectors in a 2000 B16 and when you convert them with a resistor box you need to connect them to the injector wires that lead to a plug on the back of the engine below the intake manifold also know as a dead end plug. This may be where my problem is since my interior dash lights seem to dim and pulse with the throttle and honda has issued a TSB for checking this plug and re-wirelooming them to ensure none of the wire get damaged or worn and can short out fuse 15 in the underdash. Could also be getting to intermittent problems with dead end plug #2 which is for the injector wires....