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
Assuming HC/CO is ok, If your car has an EGR system - this is the first thing to check. The EGR reduces combustion temperatures. NOx is the result of too high combustion temps. Look at all the factors that raise combustion temps. If car is high mileage, consider a de-carbon treatment. Cooling system - make sure it's not running too hot. Timing - not too far advanced. Lastly, yes, the O2 needs to switch fast. Very lastly, consider the cat. However, note HC/CO/CO2 when looking at the cat. The only real test on the cat for NOx is the intrusive test [checking NOx before and after the cat]
James
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