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I'm not an emissions expert, but have been doing a lot of research on an unrelated V12 problem. Anyway, if I understand what I'm reading from Bosch Fuel Injection & Engine Management manual (page 2-16), it suggests elevated NOx emissions can result from too lean an air-fuel mixture and elevated combustion temperatures. The section discusses the balance that needs to occur between too rich (elevated CO and HC emissions, but lowered NOx) and too lean (elevated NOx emissions, but lowered CO and HC). Apparently, there is a happy medium. Maybe you're just a little too lean.
Not sure if this will hinder or help, but it should give you a lead on where to go next - air-fuel mixture ratio. Good luck.
Cheers,
Jeff
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