Toyota Repair: 90 Corolla Emissions Failure (NO2), nox emissions, combustion temperatures

Question
You are correct.  It is the NOX reading that caused the failure.
Thanks,
Greg
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Followup To
Question -
Ted,
My 1990 Corolla recently failed the Ohio emmissions check for a higher than acceptable level of NO2 (about 2/3 higher).  What is the most common cause of this failure, what is the likely repair required, and is it something that I can repair/effect myself?
Thank you,
Greg Gallatin
Answer -
There is no such thing as NO2, please check again, could it be you meant NOX, CO2,CO or O2?  

Answer
High NOX emissions are usually caused by high combustion temperatures, this can be caused by an exhaust gas recirculation(EGR) system failure, this is the first step in diagnosing this condition, have someone check to make sure the egr system is functioning properly, the egr system was specifically designed to reduce NOX emissions.
It can also be caused by incorrect ignition timing or engine overheating,If you wish you can give me the rest of the readings(HC,CO,O2)this would help me try to make a more accurate diagnosis.