My '93 Miata marginally failed the TX State Inspection for NOx at the high
speed and low speed. I changed the spark plugs and air filter (was due for
this anyway) and had it tested again - this time I passed the low speed test
(barely) and still failed the high speed test (1,562 ppm NOx vs the standard
of 1,179 ppm). Everything else was in great shape.

The guys at the shop suggested the next step would be to change the O2
sensor. Does that make sense? Any other suggestions? Thanks.

Nick