Emission Gases Explained Pt.1

Pt.2 / Pt.3 / Pt.4 / Pt.5

8) High carbon dioxide (CO₂) readings indicate a nearly ideal air-fuel ratio and efficient combustion

9) Low carbon dioxide (CO₂) readings indicate a fuel mixture either too rich or too lean, exhaust system leaks, or sample dilution.

10) Oxides of Nitrogen readings. Oxides of nitrogen (NOx), including nitric oxide (NO) and nitrous oxide (NO₂), are formed if the combustion temperatures within the combustion chamber exceed some 2,500 degrees F. This can occur when the engine is under load. When excessive temperature conditions exist, the greatest amount of NOx is typically produced at the stoichiometric point (AFR 14.7 or Lambda of 1.0) as the engine is under a light load. If the combustion process within an engine is burning fuel at or near stoichiometric point, NOx levels on acceleration will typically read significantly higher than those measured at cruise and during deceleration. Typically, the NOx readings at idle will be 0 PPM.

11) High NOx Readings. Circumstances that can lead to abnormally high NOx emissions are:

* Malfunctioning EGR valve

* Lean fuel mixture (AFR above 14.7, Lambda above 1.0)

* Improper spark advance

* Thermostatic air heater stuck in the heated air position

* Missing or damaged cold air duct

* Combustion chamber deposits

* Malfunctioning catalytic converter

* Catalytic converter intervention and NOx concentrations
High NOx readings at the tailpipe are an clear indication that there is a problem in at least one part of the system, but an NOx reading that appears within "normal" ranges or is only modestly elevated is not necessarily a reliable indicator of proper or even acceptable system performance. NOx readings at or near "normal" are possible, and not uncommon. From a malfunctioning engine equipped with a properly functioning catalytic converter. In such circumstances, truly elevated pre-catalytic converter NOx levels will be masked by the catalytic converter and the potential for an NOx problem must be further evaluated in the context of other readings of abnormal gas concentrations and AFR / Lambda readings.

12) Low NOx readings. There is, effectively, no reading for NOx that can be characterized as too low or below optimal. NOx is naturally 0 ppm at idle. NOx concentrations may appear normal even in a rich burning environment where the AFR is well below 14.7 (Lambda below 1.0).