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1994 Toyota Pickup 2.4 liter failed for high NOx.

My truck failed smog yesterday for the first time ever. It has never even come close to any Max number on any test before this. It failed only on high NOx at 15mph (981 ppm) while all other results were way below Max. Results at 15/25: RPM-1638/2508, CO2-14.41/14.86, O2-0.62/0.04, HC-15/29, CO-0.01/0.09, NO-981/54. I've been to three Test & Repair stations, showed them the test results, and each one told me with confidence that they knew what the problem was. But they all had very different presumed diagnoses!! I am so afraid of getting sucked into some expensive cycle of trial-and-error diagnostics by non-experts that I decided to reach out for some expert help. I don't mind paying for the correct repair and I truly want my truck to stay way under the emission limits, but I have no faith in these people. Can you help? Thank you in advance for any experience/expertise you can provide.
Answer:

Two things you want to look into: 1. The EGR system. You need to ensure the EGR valve is opening under vacuum and allowing the combustion chambers to cool down; reducing NOx. 2. Fuel Feedback System. Your Nissan needs to manage fuel delivery correctly. It is possible your truck's engine is running lean and causing high NOx. We can only suggest that you find a smog check station which will guarantee their diagnosis. The best smog stations are Gold Shield certified. posted by SmogTips Support

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