I know this has probably been addressed here before, but I thought I'd share one of my few successes related to the NYC dyno emissions test, which I just passed today. The car (a 93 945t with 265k miles) had initially failed the last couple of years in a row, only to be 're-tested' and 'made to pass' (don't know how they did it, suffice to say that on one of these tests, the NoX printout was actually 0.0, I think an impossibility!).

In any case, this time it passed on its own merits by using the following 'remedies':

-new oxygen sensor (even though I had no 'check engine' light, this was the original sensor with 265k on it, so I figured it must be shot by now. Sure enough, when it came out, it was caked in ash and other crud from 17 years of daily driving. The OEM Bosch from EEuroparts was just over $100 and I immediately noticed much better throttle response, smoother idling and slightly better fuel economy.

-checked the bolts fastening the downpipe to the turbo exhaust port - these often loosen up, allowing external air to be sucked over the O2 sensor, which sits right there. Mine were already tight this time around, but have not been in the past - definitely worth checking.

-Mobil 1 10w30 oil change the week before. Have always used Mobil 1, and this car still has its original turbo (knock on wood) with excellent spool-up and tight response.

-bottle of Techron and premium gas fill-up a few days before, with some intermittent highway driving, overdrive disengaged. This seemed to smooth out the engine even more - for the first time in years I can now hear the nice 'whoosh' of the turbo over the engine growl when punching it.

-I should note that I did a full new wire set, cap, rotor and plugs last year before my inspection and the car still failed then - probably due to the O2 sensor and maybe valve/injector crud. So a standard tune-up is certainly also important.

-Finally, the best news : (according to the 3 mechanics I talked to before I could actually find a shop with a still-functioning dyno) - New York will switch to a tailpipe idle test for cars older than OBDII starting in January. Seems like the dyno equipment is getting too expensive to maintain, and there aren't enough cars younger than 25 but older than model year '96 to merit keeping the whole thing going. New Yorkers, let's keep our fingers crossed on this one....