Dollars to donuts it’s your catalytic converter—the car is the right age for the cat to be giving up the ghost. You should replace it with a new cat, though with a car this old you could probably go with a less-expensive aftermarket one, since the car’s probably not going to be around for the 20 years a $700-1200 OEM cat would last.

That said, the REALLY cheap ones—like >$100 on Amazon—literally only last a couple of months. (Don’t ask me how I know.)

Driving it like it is now won’t hurt anything (except the environment); it just means you won’t pass a smog inspection if your jurisdiction requires them.

If you’re reasonably clever with basic electronic stuff (and also a slightly ethically dubious person), you can solve this problem permanently for a few bucks by hacking the downstream O2 sensor circuit. This is illegal and probably morally wrong because it just hides the fact that your car is out of compliance with the smog laws, but if you Google “P0420 O2 sensor hack” someone will tell you how to do it.

There’s always a chance of aftermarket parts being bad straight from the box, in this instance however that’s probably not the case.

The fact that the ECM set a p0420 means the O2 is functioning, in all likelihood the cat was bad already the ECM just didn’t know it due to the O2 sensor failure.

Well it’s a Honda, they don’t like most aftermarket sensors. Because it was a heater code I would suggest an OE sensor first. It’s possible the cat went bad while the light was on but if the heater code was the only code I would suspect it doesn’t like the sensor.