OBDII vehicles do not have enough different gas sensors to be used to actually measure emissions. The system uses the pre and post catalysis oxygen sensors to judge the effectiveness of the catalytic converter. The assumption is that if the oxygen sensors are "in sync" that the converter is not working to convert the "bad gases" to benign gases. You can look at the oxygen sensors and make a pretty good judgement on the CO and excess hydrocarbon emissions, but there is no sensor for NOX. I have a scanner that has a five gas analyzer on it, so I can read the ODBII computer values at the same time as I see the exhaust gas. That is the only way that I know to check the emissions.
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How to measure emissuion using OBD
Re: How to measure emission using OBD
