I have a 98 Nissan Sentra SE.  I just took it down for its last
emissions test (Colo. Springs is no longer doing these tests after the
end of the year due to improved air quality).

Emission test results:

Odometer:  153248 miles
               Idle       2500
HC PPM          0          0
CO %            0.00       0.00
RPM             806        2382

Catalytic Converter --    Pass
Fuel Filler Restrictor -- Pass
Oxygen Sensor --          Pass
Check Engine Light --     Pass
Fuel Cap Results --       Pass

Overall Result:           Pass

The only maintenance I've had done on the ignition/emission system,
besides what is recommended in the Owner's Manual, was replacing the
spark plug wires  -- a few years ago, it was starting to run a little
bit rough at idle, and I took it down to the shop I usually go and they
hooked it up to a scope, and said two of the spark plug wires were bad.

Just thought I'd pass my comments along -- in this day and age, its not
too often I get to make positive comments about products.

Everyone have a nice weekend --

Jim