P0420 occasionally Only when stepping on it

Okies, during my trip to KS City and back to LA my car threw the code like 6 times.
Since I have been back to Los Angeles like 3 1/2 weeks not once.
But it has happened sporadically in Los Angeles, before the trip as well.
I read some post here and well I do have the cable and laptop, read about some tests you could do to track it down, but I couldn't quite understand the poster.
I could use some detailed instructions so I can at least know what the problem is might be a relatively cheap fix.
Getting the check engine light is always quite unnerving, like to eliminate this cause if it doesn't cost me an arm and a leg.
Help
terramir

 

1. No way to exactly know, because the odometer was replaced but here's my estimate 140k miles in Feb of '14 (service record) totaled in August of '14 with 103k I did the exact math between the exact numbers and it was 36300 missing miles plus what ever miles the original owner drove between feb14 and the accident Since he put 140k miles between Oct 03 and Feb 14 about 6k more. Current odometer is 141k plus 42k is 183k.
2. Some emissions service was performed at 100k miles heck if I know what though.
3. But emissions services are covered under warranty till 150k miles (California car CARB )so that could have been a cat replacement for all I know.

 

Yeah That's what I need detailed instructions how to run a test (s) with the mini VCI to determine what part is failing
terramir