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Excess NOx is usually caused by high combustion temperatures. But with your car in such good tune and the fact that NOx was close, I doubt that is the cause.
Usually when you fail only NOx, and its close, its your cat. I remember running into a lot of posts about the Magnaflow cats when I failed emissions a year back. They are good for 2-3 years tops, before they need replacement again to be effective.
Here are my smog numbers for reference (original cat):
First E-Test after Purchasing Car (2/23/2008)
HC - 1.6 (Limit 2.5)
CO - 15.6 (Limit 20)
Nox - .017 (Limit 3.5)
First E-Test Failure (2/26/2010)
HC - 3.2(Limit 1.5)
CO - 120(Limit 20)
Nox - .024 (Limit 3.5)
Latest E-Test (3/26/2010)
HC - 1.3 (Limit 1.5)
CO - 5.6 (Limit 20)
Nox - .094 (Limit 3.5)
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