Causes of High NOx Emissions

High NOx emissions can occur when an engine's air-fuel mixture is too lean. This can happen when too much air is added to the fuel or when too little fuel is injected into the engine. In either case, this causes the mixture to burn hotter. This could be caused by a problem with the vehicle's oxygen sensor, malfunctioning air flow and coolant sensors or a leak in the fuel system.

For maximum running efficiency, the motion of the pistons inside an engine's cylinders are precisely timed to capture the most energy from the expanding gas. Older vehicles often have adjustable timing, which allows it to be set improperly. This will lead to improperly burned fuel that creates higher NOx emissions.

A vehicle's catalytic converter helps remove NOx from the exhaust before it gets to the tailpipe by passing the gases through a titanium oxide membrane. A defective catalytic converter can then result in elevated NOx emissions. This is especially a problem in vehicles that don't have exhaust-gas recirculation valves, since they rely more heavily on the converter to deal with NOx emissions.

Exhaust gas recirculation systems route a small portion of a car's exhaust gases back into its intake manifold. This replaces some of the fuel-air mixture with the inert exhaust gases and lowers the combustion temperature. Since NOx is created at engine temperatures of 2,500 degrees Fahrenheit and above, this reduces NOx emissions. If the EGR valve, passages or control system are not functioning properly, emissions will go up.

Engines with more than 150,000 miles on them may have deposits of carbon built up inside their chambers, cylinders and valves -- especially if the engine has been badly maintained. This leaves less space for the fuel-air mix to expand, which results in it burning hotter and under higher pressure. This produces higher NOx emissions. These deposits often can be removed by a mechanic at an emissions testing center. If all other possible causes of high NOx emissions have been ruled out, carbon buildup is the likely answer.

Engines have coolant systems that circulate coolant fluid through the engine block to carry heat to the radiator. If these systems (or the temperature sensor that controls them) are out of order, the engine will more frequently heat up to temperatures at which high levels of NOx are generated.