2013 boss302 po420 code came on. What to do to fix it?

 

Google will find lots of links with information on that, such as this one:
P0420 Code - Meaning, Causes, Symptoms (Simple Fixes) (mechanicbase.com)

Since the catalytic converter itself is very expensive, I'd look at the O2 sensor(s) first

 

Yessir, Bert, it could be the downstream (post cat) passenger O2 sensor, and that's a relatively minor cost to replace. Relatively being relative to relatives and all. Your mileage may vary. And all things being fairly copacetic, it'll be that most likely.

However, one should do diagnostics on this before just aiming and firing a parts cannon. It could be the wires on the harness side, a ground, an exhaust leak at the manifold, and of course the catalytic converter itself.

Now, if it's the converter... what kills a converter? Trauma, like it's been hit with something and the substrate in it is broken up, or more likely... there's a leaky injector that's causing extra fuel to get in the cat, which burns in the cat like a furnace, and melts the substrate. This is what happened to my '97 HoopTBird, the injectors just decided to stay open a lot when off, and all three of those cats died a short time after I'd replaced the injectors.

You would think that the computer would report this, however... if the car's otherwise running right, and let's be honest the Boss 302 isn't exactly gonna be tuned for supreme emissions standards upkeep, the little bit extra that this or that cylinder might get wouldn't necessarily be noticed. The way the thing works is based on Oxygen detection, not extra fuel detection. And it's not very bright either. It figures if the O2 is in about the right place, the fuel trim (aka, injector opening timing) is right and all is well. There's no need to adjust the fuel. But the injector leaks a little bit, and over time... well, there ya go.

Theory of mine, but yeah. Check them injectors, should you find the cats have died. Which I sincerely hope has not happened.

Hope that helps, and welcome to the forums!

 

Start with checking the 02 sensors first, followed by the 02 wiring harness.. If both check out okay, then check for any exhaust/intake/vacuum leaks.. If the codes come back afterwards, only then would I consider suspecting the catalytic converter itself as being the cause.. However, I would look into getting your car properly diagnosed by a reputable auto repair center first before forking out $$$$ towards replacing your factory catted mid-pipe

 

Your very welcome!