Ok, so I just moved to CO and went and took an emissions test today so I could get the mach registered. Well, the mach failed its test because it seems the NOx emissions are too high. Limit is 3.00, failed with a 3.79. Everything else was well within limits. Car only has bolt-ons and a tune (done in AZ), and it is not even a really aggressive tune.

So, how do I go about getting the NOx emissions down? Their form states that "High NOx readings are the result of high combustion temperatures and/or high combustion pressures." So, would switching to a colder set of plugs help? Running a set of Denso's right now that were put in when the tune was done (about a year old now), can't recall if they were stock heat range or not though.