I know this is pretty much a long shot.

Does the Temperature have any relationship to efficiency of the cat that it would effect the reading at the rear sensor.

To the point it would trip the MIL??

Does the ECM see a major difference from driving through a puddle no doubt several hundred degree temperature change and the spray would also hit the sensors as well.

All This within the last 5000 miles

2 New Cats from Toyota two new sensors fore and aft Denso along with a New MAF ( toyota) New Air Intake hose New Air Filter Housing Might have missed some things.

Still Get a Random PO420 Could be a few hundred miles might be 10.

Which has me thinking about a major temperature change causing this headache.

 

I wonder could a change in the brand of gas some how degrade the cats performance.

I kinda fuel up when I need it most time the same places but sometimes I end up going From point A to D

It has been good all last week temperatures from almost warm 40 degrees F to down around 0 degrees F

I really hate not knowing the cause of problems.

 

Absolutely. It has a direct relationship. A cat that's not hot enough will not do anything, and the O2 sensor will know.

 

Perhaps I just got one or more of the Cats built the day after some major Holiday when most on the line building cats called in sick.

I guess the only thing I can do is try another two cats How does one prove they are bad?/

Got the front and rear sensors again thinking they had gone bad still getting the intermittent issue at random times.

Despite how the Plugs are sealed against weather I wonder if the winter chemicals are causing problems.