Q: My car failed the emission testing at the inspection station. The inspector said the engine exceeded the acceptable oxides of nitrogen. What could cause these high levels?
A: High NOx (oxides of nitrogen) levels can be caused by incorrect ignition timing (common cause on older vehicles), faulty EGR (Exhaust Gas Recirculation) system, engine running lean (air-fuel ratio), engine overheating, carbon deposit buildup in the combustion chambers, or a defective catalytic converter.
NOx stands for the level (in grams per mile, gpm) of oxides of nitrogen. The meaning of the test, is that your vehicle emissions of nitrogen oxide are too high. This can be caused by anything that can rise the temperature of the combustion, most commonly a problem in the exhaust gas recirculation system. High NOx emissions could be also the effect of many other problems, such as problems in the computer control system or in the three-way converter.
Harder to diagnose are elevated oxides of nitrogen (NOX) emissions. Causes here may include a defective EGR valve, EGR vacuum solenoid or motor, plugged EGR ports in the manifold, over-advanced ignition timing or engine overheating.
Health effects
NOx react with ammonia, moisture, and other compounds to form nitric acid vapor and related particles. Small particles can penetrate deeply into sensitive lung tissue and damage it, causing premature death in extreme cases. Inhalation of such particles may cause or worsen respiratory diseases such as emphysema, bronchitis it may also aggravate existing heart disease.
NOx react with volatile organic compounds in the presence of heat and sunlight to form Ozone. Ozone can cause adverse effects such as damage to lung tissue and reduction in lung function mostly in susceptible populations (children, elderly, asthmatics). Ozone can be transported by wind currents and cause health impacts far from the original sources. The American Lung Association estimates that nearly 50 percent of United States inhabitants live in counties that are not in ozone compliance.
NOx (especially N2O) destroys ozone layer. This layer absorbs ultraviolet light, which is potentially damaging to life on earth.
NOx also readily react with common organic chemicals, and even ozone, to form a wide variety of toxic products: nitroarenes, nitrosamines and also the nitrate radical some of which may cause biological mutations. Recently another pathway, via NOx, to ozone has been found that predominently occurs in coastal areas but of note is that not only is ozone produced directly from NOx but the much more insidious Chlorine radical pathway as well.
Hydrocarbons:Elevated hydrocarbon (HC) emissions usually indicate ignition misfire due to fouled spark plugs or a bad plug. But high HC emissions can also be caused by burned exhaust valves (check compression), lean misfire (check for vacuum leaks, low fuel pressure or dirty injectors), or rich fuel conditions (fuel saturated carburetor floats, excessive fuel pressure, leaky injectors or a dead O2 sensor).
Carbon monoxide:High carbon monoxide (CO) emissions are a telltale sign of a rich fuel mixture. On older carbureted vehicles, fuel-saturated plastic floats, incorrect float settings, leaky power valves and misadjusted chokes are often responsible for the rich mixture. On newer vehicles with feedback fuel controls and fuel injection, leaky injectors, excessive fuel pressure and sluggish or contaminated O2 sensors are all possibilities to investigate.