California Smog test!



Hi, I recently moved to CA and had my '93 Toyota Corolla smog tested. It passed the HC and CO emissions and was even under the average. But it FAILED the NO emissions at the slower speed of 15 mph and barely passed the NO at the higher speed, 25 mph. The garage performed a diagnostic test looking under the hood and then under the car and said I needed a new catalytic converter. So I went ahead and got a new cc put on my car. Great news my car passed! The emissions at the slower speed dropped considerably and was now within the acceptable amount but still much higher than average. BUT the amount at the higher speed of 25 mph only dropped 2-3 points. The results seemed a little odd to me that the new catalytic would affect one speed but not the other. I’m just trying to understand what this smog stuff is all about and want to make sure the garage didn’t do something fishy. I’m getting mixed messages from the people I’ve talked to and would like to understand this stuff better. Should a new catalytic converter help drop the NO emission levels at both speeds or are the results I got reasonable??

Thanks for any help or advice you have for me!


The cat will help under low speed conditions but not at higher speed condition for NOx. It’s the job of the EGR circuit to control NOx emissions at higher speeds.

The EGR system allows a small amount of exhaust gas back into the combustion process. This gas reduces the combustion temperatures in the cylinders. This then reduces the NOx emissions.

So what should be looked at is if the EGR circuit is operating correctly.



The converter plays a relatively minor role in NOx emissions. The EGR system does the heavy lifting with NOx…Also, ignition timing plays a considerable part with NOx. Too much spark advance causes NOx to soar…

The emissions controls on your '93 were still in the development stage so consider yourself lucky and drive on…


The EGR system was DESIGNED to lower NOx (nitrides of oxygen). Timing decreases helped, also.
You need to insure that the passages for the egr are clear, and allowing unrestricted flow into the intake manifold. This involves cleaning the egr valve, and the passages to and from it. Check the operation of the egr system (instructions in the repair manual).
Since timing may be a bit high (which would increase NOx), check and set timing at specs.


Thanks for your reply.
It makes sense about the ERG system…
I’ve been told that the converter can play a role with lowering emissions at slower speeds but not so much with the higher speeds. Would you agree??


I’ve been told that the converter can play a role with lowering emissions at slower speeds but not so much with the higher speeds.

I believe that is true, but the converter does little for NOX. Also note I believe that the emissions the converter does work well on, at at low power when the amount of those emissions are high.


ok, but if the converter does little for NO emissions why would my NO reading for 15mph after the converter was replaced be significantly lower?? The original reading was 1062 and with the new converter it is 630. Do you think the new reading is legit or did the garage pull a fast one on me??? That’s what I’m trying to determine… Did I really need a new converter to pass the low speed NO emissions??