I'm not familair with your state's emission readouts the way they list them is different than Texas but I can tell you that NOx is controlled by the EGR and that a couple of things that happen on high mileage Toyotas is that the vacuum switching valve (VSV) gets restricted and the EGR modulator gets plugged up. Both of these items are easy to check.

This part you may not like, I have had Supras with "high flow catalytic converters" that would not pass a sniffer test, we removed the high flow piece and replaced it with a borrowed stock cat and it passed no problem.

Also short the Te and E1 terminals and then check and adjust (if needed) your timing, this is also easy to do and can have a big impact on your readings.

I wouldn't worry about the headers if they've never been a problem before.

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There is a fine line between a hobby.... and mental illness.