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Have you tried replacing those nasty ass plugs with a good set of factory Volvo ones? NoX is always the by product of heat, you need to figure out why it is running hot enought to form too much Nox. If it is slightly elevated it could be entirely normal and a new cat and O2 sensor will address it. Most often times this is not the case. Things like lean air fuel mixtures, restricted fuel filters, biased rich 02 sensors, carboned up combustion chambers causing high compression, faulty pre-heat thermostats, these are some of the usual culprits in a high Nox case. Chances are you could have a few things going wrong. Depending upon how high it is you could probably hang the new cat and O2 sensor and get it through the inspection, the problem with that is you will have not addressed the root cause of the NoX pollution and it will burn up that cat in short order and the next time you are up for an inspection it will be back to square one minus the money you spent to make it clean for a day last time. My advise is to seek out the services of a competent, honest, Volvo smog technician, good luck on that one.
Mark
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