Your 1996 Honda Accord high NOx failure might be caused because of a defective EGR valve and/or system. High compression, high engine temperatures, and/or vacuum leaks which cause lean fuel mixtures (low CO) will also cause your 1996 Honda Accord to produce high NOx. If after all the emissions systems on your Honda Accord are inspected and no faults found, consideration should be given to a defective Catalytic Converter.
We recommend inspecting your Honda Accords EGR valve first. The EGR valve is responsible for re-circulating a small amount of exhaust gas back into the combustion chamber. This is done to cool down the chamber temperature by producing an inert gas along with the regular fuel mixture, keeping temperatures under 2500 F, and lowering NO.
Your Honda may very well need an EGR cleaning, but you also want to make sure the engine is in proper fuel control, and is not running lean. If CO is below .05 during the 15mph or 25mph portion of the smog test, this may indicate a lean fuel condition. If the EGR system is working fine; the EGR valve opens when vacuum is applied and once it opens (while your Honda Accord's engine is idling) the engine idle goes down to almost stalling the engine, the EGR system is working. The only thing left to make sure at this point as it related to the EGR system is to ensure there is vacuum coming from the intake manifold to the EGR control valve.
For problems which could cause your 1996 Honda Accord to fail the emissions test for high follow the link below:
What Causes high NOx?
