If your Hyundai Accent is equipped with the 1.5 liter engine it more than likely does not have an EGR valve, which means... NOx reduction is completely up to the catalytic converter, and more importantly, proper air/fuel mixture.
So the first thing you need to do is ensure that your engine is NOT running lean, in other words, make sure enough fuel is getting to the combustion chambers. A lean running engine is a hot running engine; and NOx is created when an engine runs hot, hotter than 2500f engine combustion chamber temperature. Keeping the engine cool by running a proper (14.7:1) air/fuel mixture keeps NOx low.
You'll need to check for vacuum leaks, proper oxygen sensor operation, proper mass air flow sensor operation, and proper fuel injection, as well as any problem which might cause a fuel restriction such as old fuel filter.
Diagnosing your Hyundai Accent's NOx failure will take some engine emission operation know-how. We recommend a certified smog check repair station conduct a diagnosis. A smog repair station will usually charge 2 hours of labor for smog failure diagnosis, and the average hourly labor rate is between $50.00 to $75.00.
Read more on what causes high NOx emissions here.
