OBD II code PO420

[astrojohn]

I have a 2000 Astro AWD, with 311000 miles. It is in great shape, and runs and drives better than the 2 I had previously.
I have not had any issues with the motor, except I keep getting a code PO420. I have my own scanner, so when the light comes on, I can check it out.
The thing is, it only happens when I'm in a town or city, where I get to a stop light, and have to wait for a bit, for the light to change. Then it comes on. Driving to and from work, it doesn't come on at all, just stop and go situations.
I've replaced the rear O2 sensor ( the one by the cat converter) as what I thought I read, would be the one causing the problem.
I'm not against replacing the others, but the definition of the trouble code is vague, which also includes replacing the cat converter.
I don't think the converter is plugged, as the exhaust isn't hot coming out of the tailpipe, nor does the cat glow in the dark.
I did at one time only have another O2 sensor code, which I can't remember, but was something about the heated O2 sensor not reading. But it only happened once, and it came on withing 2 minutes of starting in the morning.
Anyone out there can enlighten me I would appreciate it. I'm tired of resetting the check engine light ( I know I shouldn't be doing that, but am not sure what to replace for sure).
Thanks for any and all answers.

[v8famvan]

Since this is the only code that you mention that you getting, you will need to replace the cat converter - converter catalyst is degraded & ineffective. The system will only run the cat converter efficiency monitor test (P0420) if all the O2 & other sensors are working good.
For example: if you had a faulty mass air flow sensor & it was causing a code, the P0420 would not run to report a pass/fail. Since the mass air flow is not setting a code at this time, you should assume it is working good along with all the other sensors that are not setting codes.
With over 300,000 miles, the cat has given a good lifetime of service.