I tend to disagree with that
I’ve encountered several older vehicles which burn on oil, aren’t running rich, in fact the engine is and always was in perfect mechanical shape. yet the catalytic converter can NOT clean up the emissions enough to pass the smog test
Now let me be more specific . . . these vehicles I’m thinking of don’t have absolutely wasted, plugged up cats, which are literally slowing it down on the freeway
They drive just fine, power is normal, no smell, they are no longer effective at cleaning up the emissions well enough to pass that smog test
the before and after results of the 5 gas analyzer also prove it
But what do I know . . . ?