What causes high NOx (nitric oxide)?
High NOx is caused by high combustion temperatures and pressures.
Problems that can cause high NOx include:

   * An inoperative EGR system. The EGR system was designed
specifically to reduce NOx; that's it's only function. The EGR, which
stands for Exhaust Gas Recirculation, allows exhaust gas to enter the
combustion chamber through the intake. The exhaust gas has a cooling
effect on the combustion chamber.
   * Over advanced ignition timing. The more advanced the ignition
timing, the higher the combustion chamber temperatures.
   * Overheating. If the engine temperature is too high the NOx
emissions will go up.
   * Lean air/fuel mixture. If the air/fuel mixture is too lean the
combustion temperature will go up.
   * Compression over specification. Normally when we think of engines
aging we think of the compression going down. However, there are a
couple of ways the compression can increase as the engine is used. One
is carbon buildup on the tops of the pistons. When carbon forms on the
tops of the pistons, the combustion chamber area decreases and the
compression ratio increases. The carbon can be manually scraped off the
tops of the piston to correct this problem, but that would require
disassembling the engine.
   * Mysterious reasons. Sometimes NOx is too high and there's no
obvious cause: the EGR works, the timing and advance work normally, the
engine temperature is within range, the air fuel mixture is good, and
the compression is within spec.. When this happens you can try a little
tweaking. If there are two thermostats listed for the car, use the
cooler one. If there are two spark plugs listed for the car, use the
cooler one. Retard the timing 2 degrees (the maximum allowed). Clear
out the EGR passage even if the passage seems fine. Cross your fingers.

My 1993 9000 non-turbo failed the CO test recently.

Turned out to be a defective vacuum-hose going to the fuel pressure
regulator!

Simple fix.

Frode

just got my 1987 which had been difficult the last two years to pass.  this
is what I did

1. put it one heat range colder plugs.
2. replaced the O2 sensor.
3. replaced the CAT

It passed easily.  should have done this last year...

I'm in California too!

the 1990 has the LH2.4 or 2.4.1 so it should be easier to pass given a
recent tune up, air filter replaced and the three things above.