perhaps you should consider turning off your vin diesel mods before getting the inspection.

I had exactly the same problem with my 94 turbo. The CO2 and Hydrocarbons were stellar but NOx failed miserably. If your car is equipped with it, clean the EGR valve (not all cars were equipped with it, even of the same model and year!). Since mine was not equipped with an EGR system, after flushing the cooling system and cleaning the oil trap and hoses, the only other option was replacing the O2 sensor (the on board diagnostics reported a code indicating "slow O2 sensor response"). The O2 sensor alone brought the NOx from 793 to 47 ppm and more importantly, a pass. In every other way the car was working well before the O2 sensor.
Good luck

If you show us the results of the testing we may be able to tell more. Usually high NOx goes with low HC and CO. Is this the case with your test? Could be as simple as an air leak somewhere, could be as costly as a new cat. Don't awfullize and check the easy and cheap stuff first.

High NOx means that either there's too much oxygen in the air/fuel mixture, or the ignition timing is too far advanced. I don't know the ignition system on the 95 car, but I'll bet that timing is not a problem.

As for the mixture, a common cause of a lean mixture would be a vacuum leak. Nine year old vacuum lines could certainly have such leaks. That's where I'd be focusing my efforts: looking for vacuum leaks.

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Jim Rothe, '99 S70 T5M,
http://www.jimrothe.com/volvo/index.html