The p0420 code few questions

I have a 2000 2.4 Tacoma code just came one. There are no exhaust leaks and the truck still runs great and I'm getting around 26-27 mpg so no loss in gas mileage. What should I start with what should I replace first I tried resetting and putting different gas popped back on about 70 miles later.

Also I looked at the o2 sensor it was pretty dirty old looking but there was no plug I've changedno2 sensors before but there was always a plug that you just unplug this one has no plug so how would I change it and which o2'sensor do I change for this cod ethe one before the cat and after the manifold or the one after the cat and before the muffler.

 

Either bad sensor or bad cat. Just have to narrow it down.
What do you mean by 'no plug'? Not connected to the harness?

I think you can test the sensors with a resistance check.
If either one fails that, then replace it.

 

Yea there is no connecting harness on the o2 sensor behind the cat only on the one before the cat

 

Well, that has got to be the problem, then.
Is it possible that someone tried to steal your cat, and cut off
that wire, before giving up or being scared off?

If so, you need to re-establish the connection. There should be dangling
wires (one from the sensor, and one from the harness nearby).