Not to be simplistic, but I had all kinds of close to limit non-passing readings at a couple of emission test workshops in stringent California and I wound up getting a new aftermarket catalytic converter and passed with flying colors. (This after timing tweaking, new O2 sensor, filters, oil change, etc.) If yours is old, consider a change and you can end the inorganic chemistry mystification. Them cats get loaded with byproduct too. Good luck