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Examples of Deceiving Post-Catalytic Analysis
When troubleshooting an emissions failure, your primary concern will be what comes out of the
tailpipe. In other words, it doesn't matter whether the efficient burn occurred in the engine or the
catalyst. However, when troubleshooting a driveability concern, the catalytic converter may mask
important diagnostic clues which can be gathered with your exhaust analyzer. The following are
examples of situations where post-catalyst reading may be deceiving.
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Example 1
: A minor misfire under load is causing a vehicle to surge. The exhaust gas
from the engine would show an increase in HC and O2, and a reduction in CO2.
However, once this exhaust gas reaches the catalytic converter, especially a relatively
new and efficient catalyst, the oxidation process will continue. The excess HC will be
oxidized, causing HC and O2 to fall, and CO2 to increase. At the tailpipe, the exhaust
readings may look perfectly normal.However, once this exhaust gas reaches the catalytic converter, especially a relatively
new and efficient catalyst, the oxidation process will continue. The excess HC will be
oxidized, causing HC and O2 to fall, and CO2 to increase. In this example, it is interesting to note that NOx readings will increase because of the
reduced carbon monoxide and increased oxygen levels in the catalyst feed gas. This
could be detected with a five gas analyzer. Wouldn't this create CO...isn't the CO2 only created after the nox is split and the o2 is joined with the CO2 in the catalytic converter...I mean unless it's happening right then and there? Well then wouldn't this mean there then IS enough CO for the nox to split with and the freakin nox readings should then go down like it's supposed to? Or does it completely burn and create oxygen and co2? I do notice a lot of water coming out of the catalytic converter end flange...
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