My first question is, does the CEL light come on when you first turn on the ignition? Sellers have been known to disable, also in California the seller is responsible for smog.
That being said, I'd check with a scanner for codes. High N0x is caused by high combustion temperature. This can be from lean mixtures or advanced ignition timing.
Causes could be MAF, injectors or fuel delivery, air leaks or 02 sensors. Might pull a few plugs and look at color.
Good Luck
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