NOx is created when an engine's combustion chamber temperature reach over 2500F. Could be caused by vacuum leaks, fuel control problems, engine overheating, and /or carbon build-up (high engine compression)
The fact your Hyundai did not fail for high HC or CO does not necessarily mean HC and CO are at optimum levels. A little bit of CO (unspent fuel) is needed in order to keep NO levels low. So the first thing to do here is to make sure your Hyundai Accent is in proper fuel control. In order to control NO your car should be running 14.7/1 Air/Fuel ratio. The two leading faults which cause vehicles to produce high NO is a defective EGR system, and the second is defective fuel control. After ensuring the EGR system is functioning you'll need to make sure adequate fuel is getting to the combustion chambers, and there are no vacuum leaks; adding un-metered air to the intake. If any air enters the combustion chambers (through vacuum leaks) which is not measured by the Mass Air Flow sensor, your Hyundai's engine control computer will not know to add fuel to the air, thus distrupting the air/fuel mixture (leaning it out) and causing high NO.
You'll need to have a smog check repair station ensure your Accent is in proper fuel control. The diagnosis should begin with inspection of the fuel injection “feedback” system. This includes ensuring the oxygen sensor is working properly, the computer system is receiving the signals from the oxygen sensor, the computer is computing the data properly and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery to the combustion chambers.
The fuel feedback test should be conducted using a 5-gas analyzer (smog stations usually will use their smog machine as a 5-gas analyzer) and propane. Propane will be added to your Accent's intake manifold and the oxygen sensor will be measured for an increase in voltage. Using a voltmeter the smog technician should observe the voltage output of the oxygen sensor as propane is added. Voltage should increase and the ECU should in turn decrease fuel delivery through the fuel injectors.
Along with the oxygen sensor several other emission sensors need to also be inspected during the fuel feedback test such as the mass air flow sensor, engine coolant temperature sensor, and throttle position sensors. These sensors also play an important role in assisting the ECU with proper fuel control.
At the end of the feedback test it will be know whether your Hyundai Accent is failing for high NO because of an electronic problem or mechanical.
