O2  ( Oxygen )


Measured in percent of the total sample concentration of the exhaust gases.

Free O2 occurs in the exhaust when there is an excess of air in the mixture. The O2 content increases sharply as soon as Lambda rises above 1. Taken with the CO2 maximum, the oxygen content is a clear indicator of the transition from rich to lean mixture range, or leaks in the manifold or exhaust systems or combustion failures.

With rich mixture most of the oxygen is burned during combustion.

With very lean mixture more O2 escapes "un-combusted" so the level rises.

Oxygen, measured as a percentage of the exhaust volume, reflects the amount of gas remaining in the exhaust sample after the combustion process has taken place. Ambient O2 readings should be about 20.9%, reflecting the natural amount oxygen found in the air. The ideal range for vehicles without a secondary air injection system is less than 1.5%. If there is an air injection system, O2 levels will typically fall n the range of 3% to 4%. Pinching off the air hose of a vehicle equipped with air injection should produce O2 levels similar to those found for vehicles without air injection.

1) High O2 readings indicate too lean fuel mixture (air-fuel ratio) (AFR higher than 14.71, Lambda greater than 1.0). Circumstances that can lead to high O2 emissions are:

* Lean fuel mixture (AFR above 14.7)

* Vacuum leaks, Air Injection, Additional (unwanted) exhaust dilution with fresh air

* Ignition related problems causing misfires.

* Incorrect engine valve timing

2) Low O2 indicates a rich fuel mixture (AFR below 14.7, Lambda below 1.0).


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