What Causes High Nitrous Oxide (NOx) Emissions

By Alan Yu

Your car creates nitrous oxide, or NOx, any time the internal temperatures get to 2500 degrees F. There are many systems in place designed to lower nitrous oxide emissions and when these systems fail, you will fail your emissions test. Some of the failure items are listed below:

  • Lean fuel mixture: A lean fuel mixture condition is created when the engine doesn’t have enough fuel to operate properly or it has too much air. Whichever the case, the internal walls of the combustion chamber start to get very hot and NOx is created. Some common items that can cause a lean fuel mixture are: vacuum leaks, bad air flow meter, or oxygen sensors.
  • Defective EGR System:  The Exhaust Gas Recirculation system on your car is designed to reduce NOx. It is made up of an EGR valve, EGR pressure sensor, vacuum hoses, and one or more vacuum switching valves or solenoids. Some newer vehicles may be equipped with electronically controlled EGR valves.  The EGR system's basic function is to re-route a small amount of exhaust gas back into the intake manifold to help reduce combustion chamber temperatures. As I have already mentioned, NOx is created when combustion chamber temperatures reach above 2500'F. By recirculating exhaust gas back into the intake, a small amount of the air/fuel mixture is replaced with inert gas, reducing combustion temperatures.
  • Defective Catalytic Converter (CAT):  Some vehicles operate without EGR valves and they rely heavily on the catalytic converter to assist in the reduction of NOx. These vehicles have tendencies to develop CAT problems more frequently than those which are equipped with an EGR valve. Many times this is the cause for failing the emissions test for high NOx. 
  • High engine mileage: As your car packs on the miles it also packs on some carbon buildup in the engine. This takes up valuable space needed for the air/fuel mixture to combust. It also creates higher cylinder compression which causes heat build-up and higher levels of NOx. This is usually seen in higher mileage engines that haven’t been taken care of.
  • Overheating: If your car’s engine is hotter than it should be, it is creating high levels of NOx. Have your engine’s cooling system checked and maintained regularly will not only protect your engine but also lower your NOx emission.

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