Emission Gases Explained Pt.1

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

Facts to Remember 

The byproducts of combustion are dependent on the air-fuel ratio.

13) O₂ combines with HC to form CO₂ and H₂O.

14) O₂ combines with CO to form CO₂.

15) CO is an indicator of air-fuel mixture richness.

16) HC is an indicator of fuel mixture leanness (or richness) and misfires.

17) CO and O₂ are equal at the stoichiometric air-fuel ratio.

18) O₂ and CO₂ are indicators of exhaust system integrity, sample hose and probe integrity, or both.

19) CO₂ is an indicator of combustion efficiency that peaks at or near the stoichiometric air-fuel ratios, and decreases with lean or rich air-fuel ratio.

20) Air injection systems dilute the exhaust sample with O₂.

21) O₂ is essential for proper operation of the catalytic converter. Its concentrations are essentially unchanged by the catalytic converter, providing a "window" through the converter to the engine. O₂ levels are higher on vehicles with properly operating air injection systems.

22) If CO goes up, O₂ goes down (inversely related)

23) If O₂ goes up, CO goes down (inversely related)

24) With the air injection system disabled and the CO above 1%, the catalytic converter is oxygen-starved. Without O₂, it does not fire, allowing exhaust concentrations to be more like readings taken ahead of the converter.

If readings are within the manufacturer’s or local/state/federal allowable limits, it can generally be assumed that the fuel, ignition, and emission control systems are functioning properly. If they exceed the limits, repairs or adjustments are probably called for.