NOx emissions - 1993 Dodge Shadow

Non functioning or partially blocked EGR?

These cars came both with and without EGR. The ones without EGR sometimes have a tougher time passing NOx if any NOx-increasing factors are involved, since there's no EGR to provide an additional NOx reduction.

Carbon in combustion chambers:

The 2.2 and 2.5 engines do tend to accumulate carbon in the combustion chambers, which increases NOx. Put a can of BG-44K or Chevron Techron (do not waste money on any other combustion chamber cleaner; these are the only two that actually do anything) into a tank of fuel and drive around until you're down to 1/4 tank. Then hit the highway, find a mild uphill, pull it into 2nd (if automatic -- 3rd if manual) and floor the gas. Release the gas when/if you reach too high a speed, let the car slow, and repeat a few times. Then drop back into Drive or 5th.

Ignition timing:

If the ignition timing is overly advanced, you'll have excessive NOx. Be sure the ignition timing is set to the spec on the underhood vehicle emission control information decal, and make sure the correct procedure is used (warm up engine, disconnect engine coolant temperature sensor, THEN use timing light and read timing, adjusting if necessary).

EGR:

If the vehicle is equipped with EGR, the system may not be functioning properly or may be clogged with carbon. Specific EGR service is warranted in this case.

Catalytic converter:

If the reduction section of the catalytic converter has become contaminated, melted or otherwise damaged, NOx will not be reduced. Fix is replacement of the catalytic converter.

Engine mechanical condition:

If the engine is burning oil, NOx emissions will be increased.

DS

Most likely problem, given the limited details, is a deteriorated cooling system causing the engine to run on the warm side. Second most likely is a deteriorated converter. Information as to the other levels (particularly HC and CO) would make an "educated guess" much more "educated"