Possible causes of high nox emission. on 2001 Hyundai Accent

My car, a 2001 hyundai accent failed a smog test due to high nox. What could have possibly caused it?

1 answer
nox (nitrogen oxide) form in the combustion chamber when the temperature rises above 2500 degrees f. The EGR (Exhaust Gas Recirculation) system helps to lower the combustion chamber temperature, so if it is malfunctioning it will cause high nox. These systems (EGR) often get plugged with carbon and may need to be cleaned out.