GxT V016-01, Ferret 16 GasLink II 5-Gas Analyzer - Manual (Page 19)

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Gas Theory
Carbon Monoxide (CO)
Carbon monoxide is formed when there is not enough oxygen to support
combustion. Fuel system problems are the usual cause of high CO.
In carbureted engines high CO may be caused by: Dirty air filter, Sticking
choke, Float not set properly, Idle mixture set too rich
In fuel injected engines high CO readings may be caused by: Dirty air filter,
Leaky injectors, High fuel rail pressure, Engine computer control malfunc-
tion. Carbon monoxide is an excellent indicator of rich air/fuel ratio because
it responds quickly and dramatically to changes in mixture.
Carbon Dioxide (CO2)
This is the only gas for which a high reading is desirable. Carbon Dioxide
peaks when combustion is most efficient, so the higher the reading, the
better. The usual range is from 12-15%, but the reading will fall off quickly
when the mixture is either too rich or to lean. Carbon dioxide is an excel-
lent indicator of overall combustion efficiency.
Oxygen (O2)
Ideally, all available oxygen will combine with all the fuel during combus-
tion. If there is not enough fuel to combine with all the oxygen, excess
O2 will be present at the tailpipe. For that reason, oxygen is an excellent
leanness indicator.
It is possible to have a normal or a rich mixture and still get a high O2
reading. A leak in either the engine exhaust or the gas detector’s sample
path may allow air to enter and dilute the sample. To find leaks in the
gas detector’s sample path, follow the instructions in the MAINTENANCE
section for performing a Leak Check.
Oxides Of Nitrogen
Since air is mostly nitrogen, the combustion taking place in the engine
will be in the presence of surplus nitrogen. The high temperatures of
combustion will cause oxidation of some of that nitrogen. This process
also consumes some of the oxygen that would otherwise be available for
combination with fuel components. In a properly functioning system the
resulting oxides of nitrogen (NOx) are reduced back to nitrogen in the
catalytic converter.