How is NOx controlled on your engine? What were the numbers two years ago? Do you have any other prior data to review the NOx trend?

I assume that "Euro" model means it is a gray market car. Many of them were fraudulantly certified, so understanding the specefic NOx control system (if any) and the test history of the car will be important to correcting the problem. Given that you are three times the limit, I don't think that retarding the timing alone will get it to pass assuming the timing is reasonably correct now, and if you or no one else understands the NOx control system, you're going to be on your own.

If the car has an EGR system the valve and control system should be tested. If there is no available documentation on how it works or the parts used, you or someone is going to have to figure it out.

Prior the the introduction of three way catalysts (TWC), which both oxidize HC and CO and reduce NOx, NOx control was achieved with EGR and timing calibration or some combination of the two. Prior to TWCs oxidizing catalysts (OC) could oxidize HC and CO, but not reduce NOx.

The first TWCs were introduced about the time your car was built. Cars certified to meet 49-state or CA emssions have codes on the emission/tuneup label in the engine compartment for the basic emission control equipment and will indicate if the car is equipped with OC, TWC, or EGR.

So what emission control equipment codes are listed on the label?

Duke