oxides of nitrogen happen I believe from combustion temps to high. not sure of the science but look for a vacuum leak ,too advanced spark timing , the egr, O2 sensor.
I am unfamiliar with vw's but if you can collect the codes off the computer it can help you narrow down the problem. If you have multiple codes youhave to try and play the chicken or the egg routine and figure out which sensor failed throwing off the others that are trying to compensate. Time to get out the ohm meter and check parameters.
If the are no codes then it is probably a basic<non electronic> mechanical problem ,vacuum leak, egr, distributor timing off <if possible>.
How does the car run? rough at idle and smooths out with throttle, manifold vacuum? runs good except at lower midrange rpm ported vacuum? egr?
How are the co and hydro carbon tests?