How to make sure catalytic converter is bad when you get DTC's P0420 and P0430 at the same time?

Question

I recently experienced misfiring (DTC's P0300, P0301 and P0303). The vehicle was driven for about 20 miles before it was repaired. Replaced two ignition coils and replaced all four spark plugs and valve cover gasket Gasket was replaced as oil was leaking and getting into the spark plugs' chamber.

After driving about 60 miles we topped off the fuel tank to full. Three miles later CEL came on with P0420 and P0430. After some more driving the following day the CEL went away, came back after more driving and now is off again.

Answer 1

if you are using low octane fuel that could be a cause for the catalytic converter  to go bad in the long run.

You can try to do an intake flush and use 91 octane from shell. 

Also drive the car a little hard to get the catalytic converter hot enough to burn all the carbon build up out