[Trog]

NO (PPM): Oxides of nitrogen (more commonly called NOx) are odorless gases that help form smog, and give smog its characteristic brown color. NO is produced when temperatures in the combustion chambers exceed 2500 degrees Fahrenheit. Excessive engine temperatures could be caused by a lean fuel mixture, by retarded timing, by carbon buildup inside the combustion chamber, or a defective engine cooling system.