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If I recall correctly, all 3-way cats have the reduction catalyst first and then the oxidation catalyst. NOx is controlled in the reduction end. The quick answer to your question is: The only way to tell for sure if the NOx reduction end of the catalyst is working is an emissions test.
A reduction reaction means that it separates the oxygen from the nitrogen, converting NO and NO2 gas molecules back to N2 and O2. The oxygen that is freed up in the reduction reaction is then available for use by the second half of the cat, the oxidation end, to 'burn' the excess CO and hydrocarbons to make CO2 and water.
The reduction catalyst can fail because it is poisioned by contaminants in your fuel or engine sealants. It can also fail because it is plugged with ash from oil consumption or plugged/coated with oil sludge from bad exhaust valve guide seals.
A good reduction catalyst can fail to reduce NOx if there is too much oxygen coming in to it. It cannot promote a reduction reaction in an excess oxygen environment. But the things that would cause too much oxygen to hit the front of your catalyst would have caused other problems on your emissions test.
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