Did you read the discussion of catalyst aging?

Did you read the discussion on the effects of the ignition timing map profile on NOx? Catalyst temperature?

Did you read the discussion of the ignition system, including the use of correct non-resistor plugs and checking igntion wires and cap rotor RFI resistors?

Did you read how the exhaust O2 content is a diagnostic tool? (You did not list O2 content, but the other MA report posted O2 content, so I assume your report lists it, too.)

If nothing ever changed, there would be no need for emissions testing, but components "drift" and catalysts degrade. If the duty cycle has never been checked since the car was built, it should be checked and adjusted, if required. Differences in fuel blends over the years, alone can change the duty cycle due to the addition of oxygenates and slight changes in the C/H ratio.

What I have endeavored to do in my emission discussions is to help all understand how emissions are formed and the effect of various engine operating parameters on both emission creation and control. If you spend the time to read my essays, you should gain insight into the subject and approach trouble-shooting in a rational manner in order to avoid the willy-nilly replacement of componets that could cost hundreds if not thousands of dollars in lieu of finding a possible simple problem or performing some judicious adjustements.

If you're looking for silver bullets - there aren't any.

Duke