All Honda oem parts billman, I haven't unplugged it to see if it would throw an o2 code and it just randomly started throwing the p0420 code so no mods then code.

 

Still no luck

 

Could the evap system cause this issue by a chance?

 

When mine had that code, I bought a berk hfc and a new secondary o2 sensor.

Reset the code using my obd2 scanner and I've never seen any cel since.

 

You have an exhaust leak or a faulty catalyst.

 

Whats the proper way to test for an exhaust leak?

 

Up top for help

 

Visual and audible inspections work for an exhaust leak. Every P0420 I've have ever seen on any car (I don't care who made it) built since 1996 has has a bad catalyst. O2 sensors set DTC's well before a catalyst efficiency P0420. Since you have already replaced a catalyst and sensors you have either a junk cat or a exhaust leak.

Are you 100% new, Honda sensors and catalyst have been installed?

 

100% Honda oem parts

 

Not that I doubt you, but take a pic of the cat and both O2 sensor connectors. I have a feeling you don"t have OEM parts.