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Q: NOx reading failed emissions check. on 1993 Toyota Corolla

Can I assume that if I replace plugs and wires That this will do the trick ? Or should I swap-out O2 sensor too?
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A 1993 Corolla tends to plug up the EGR system. This is common. That is why, as Bret said, that you have high NOx. I do Smog Diagnosis and Repair in Calif. and I deal with this type of stuff all day every day. The switching speed of the Oxygen sensor has some effect, but make sure that when you apply vacuum to the EGR valve at idle, the engine dies RIGHT NOW, or the system is plugged. You have dual vacuum line Egr valve and so one of the lines needs to be pinched off. It is the vacuum bleed off line.

It is not the valve and modulator to be checked for carbon, check to see if they are working properly. Check the EGR passages from the exhaust manifold, the EGR pipe, the little tube on the valve itself and all the intake manifold passages that run to the throttle body that release the EGR flow into the intake flow. These have to be really, really clean and have NO restrictions. Be able to blow through the the tube on the valve that goes to the back pressure modulator. Keep working on it, you will get. I will keep helping you. Hang in there and good luck!

I can break down my answer in a step by step manner to help you out. I can walk you through it so you can understand it. I am happy to be of help with your emissions failure and it repair.
I checked for trouble codes and got none.I 'll check the valve and modulator for carbon build up and corrosion.
It is my understanding there are two types of 1993 Corolla's. One for California and the rest of the US ("Federal"). For the life of me I can't find the EGR value on my car. Where is it?
NOx is due to high combustion temperatures, and this can be from a faulty EGR system (if equipped) or a lean running engine. Plugs and wires will cause high HC's, not NOx. Read more about NOx issues here:
Do you have any fault codes stored?
I get no trouble codes. It's 93 toyota that I can check codes with a jumper wire. Idrive less than 3000 a year .Got 177000mi. on it. Haven't changed plugs,wires,O2 sensor in a while, a long while. Guess I'll check EGR valve first. Thanks, you have been very helpful.
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