Hi Mike,
Like everyone has already said have it at full operating
temperature, I run it down the freeways at traffic speed (80) for about
twenty minutes and then let it idle to wait my turn to keep the
catalytic convert hot at sixteen hundred degrees:
http://billhughes.com/smog.jpg Of course do the normal maintenance, oil,
filters, and plugs. And I did replace the convert about ten years ago,
as that's it's job to chemically changes pollutants like carbon monoxide
(CO), hydrocarbons (HC), oxides of Nitrogen (NOx) and particulate matter
(PM) into harmless substances like carbon dioxide (CO2), nitrogen (N2),
and water vapor (H2O): http://www.tifac.org.in/do/hgt/case/convert.htm
God Bless America, ßill O|||||||O
http://www.billhughes.com/

Mike M wrote:
>
> My son's 89 6 cyl.Cherokee just barely failed the Ca. smog test. It
> passed the 15 MPH but failed at 25 MPH on the "NO(PPM)" part of the test
> (max allowed is 875 PPM.his read 1066 PPM). Anyone have any ideas what
> the problem might be?
> The mechanic told him it is probably either the O2 sensor or the
> catalytic converter.
> TIA
> --
> "Trust me, I do this all the time"
> Mike M