Here is what I found:
OXIDES OF NITROGEN (NOX)
Nitrogen makes up about 78% of the air we breathe. Though normally inert and not directly involved in the combustion process, combustion temperatures above 2500 degrees F cause nitrogen and oxygen to combine and form various compounds called "oxides of nitrogen," which is abbreviated NOX. This mostly occurs when the engine is under load and the throttle is open wide.
NOX is a nasty pollutant both directly and indirectly. In concentrations as small as a few parts per million, it can cause eye, nose and lung irritations, headaches and irritability. Higher concentrations can cause bronchitis and aggravate other lung disorders. Once in the atmosphere, it reacts with oxygen to form ozone (which is also toxic to breathe) and smog.
To reduce the formation of NOX, Exhaust Gas Recirculation (EGR) is used. By recirculating a small amount of exhaust gas back into the intake manifold to dilute the air/fuel mixture, EGR has a "cooling" effect on combustion, thus keeping temperatures below the NOX formation threshold.
On 1981 and later engines with computerized engine controls, a special "three-way" catalytic converter is used to reduce NOX in the exhaust. The first chamber of the converter contains a special "reduction" catalyst that breaks NOX down into oxygen and nitrogen. The second chamber contains the "oxidation" catalyst that reburns CO and HC.
High NOX emissions are almost always due to a defective EGR valve (or some component that controls the operation of the EGR valve). A related symptom that usually occurs when EGR is lost is spark knock (detonation) during acceleration.
Now go answer my question.