High nox can be caused by a clogged or malfunctioning catalytic converter. But before I assumed that, I would drive down the tank to empty, put diesel fuel injector cleaner in there, drive the car hard until about 1/8th a tank is left then go test again while the car is hot. A dirty intake system can cause high nox, I have seen it and also read it (a reason why so many injection cleaners claim they help emissions, they can). You may have a vacuum leak anyway. But if that were causing this problem I would expect to see the other readings off more especially if the car fuel/air were manually adjusted to work around the vacuum leak to improve idle. But these are sensitive systems so who knows what anomaly can occur in al circumstances.