Your car just failed the smog test and they said it was for something called Nox, or Oxides of Nitrogen. So what is it, and how do you fix the problem.

So let me explain it as best has possible…………….

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Nitrogen makes up 78% of the air, drawn into the combustion chamber. Nitrogen by itself does nothing when combustion. However when nitrogen is expose to temperatures above 2000 F, the Oxygen and Nitrogen atoms bond together forming Oxides of Nitrogen, (Nox).

When combustion temperatures increase above 2500 F, Nox levels also increase. This condition can occur under heavy engine loads and directly effects the flame in the combustion chamber.Under severe detonation conditions, multiple flames collide inside the combustion chamber resulting in excess Nox emissions. The only way to detect Nox emissions is using a dyno; a dyno simulates a road load condition. Nox is at its highest while the vehicle is under a load. Nox is also measure in parts per million, (ppm).

Oxides-of-Nitrogen is the 5th gas and you can measure it on a 5 gas analyzer, also you need to know Nox is better controlled when the vehicle is running slightly rich.
Here are some items that will increase Nox………..• A lean air/fuel mixture can cause an increase in Nox, because a lean air/fuel mixture creates heat.

• An Overheating engine will also cause an increase in Nox.

• A defective EGR circuit can also cause high emissions, because there isn’t enough inert gas flowing to the combustion chamber to cool it down.

• Carbon in the combustion chamber will also cause an increase in Nox, because it will raise compression ratio.

• Timing is another cause of increase Nox, if the timing is over advance Nox will increase.

• A lazy O2 sensor will also cause an increase in Nox.

• The last thing you should consider, is the Catalytic Converter, it may not be cleaning up all of the Nox like it was design to do.When diagnosing a Nox failure consider these items, most likely one of them are most likely causing the increase in Nox.

Table of Contents

Emission Page

CO Page

Emission Readings


O2 Readings

Trouble shoot CO

Trouble shoot HC

Oxides-of-Nitrogen Top Of Page


"By Nate Davis, Copyright © 2011 - 2013 Automotive-Emissions-