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My Toyota Tacoma 1996 failed smoke check: high NOx

Last Saturday I brought my Toyota Tacoma 1996 with 155000 miles to Smoke check station only to do smoke check, but my Toyota failed high NOx ( meas: 558). My question is what is part failed and how to fix, and how much does it cost? Can you give me an advise? Thank ypu very much.
Answer:

The number one cause of high Nox failures is the EGR valve. The EGR valve is responsible for re-circulating a small amount of exhaust gas back into the combustion chamber. This is done to cool down the chamber temperature by producing an inert gas along with the regular fuel mixture. Keeping the chamber cool reduces NOx emission. The EGR system is prone to collecting carbon build-up. Since the EGR valve pulls exhaust gasses from your exhaust and introduces them back into your engine's intake air over time it will become clogged or stuck open/closed from the carbon deposits. It is possible to clean the EGR valve if you remove it from your vehicle and either spray it out or let it sit in regular gasoline. We would recommend having this component inspected before all other diagnosis. posted by SmogTips Support

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