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Q: Good HC and CO but high NO on emission test on 1991 Toyota Previa

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My 1991 Previa had high HC and CO levels but good NO when first tested for emission. After refueling and added fuel injector treatment,the HC and CO levels are down (almost zero) but the NO has gone up from 433 ppm to 1939 ppm. The check engine light is also on. Could the problem be defective oxygen sensor (someone said fuel injection treatment could damage oxygen sensors) or defective catalytic converter?
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You need to find what is the reason for the Check engine light coming on, once you determine that we can get you going in the right direction. Know this, high NOx is caused by too much timing advance, non functioning or restricted EGR system, exhaust leak prior to the catalyst, engine operating temperature being too high, excessively lean mixture and finally bad reduction bed in the three way catalyst. Since your HC and CO emissions are okay, you would have to look at the catalyst, but I would not until I know what triggered the Check engine light to come on. Good luck
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