high NOX indicates your engine is running high combustion temps. Two of the major reasons for lean mixtures are intake leaks and clogged fuel injectors. The reason the engine computer cycles between rich/lean when running is because the cat needs lean mixtures to deal with HC and CO and a rich mixture to deal with NOX. If you are running lean the cat can't lower your NOX. I would first get your fuel injectors flushed and de-carbonize your intake/combustion chambers.
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