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Q: Failed smog check - NOX value @ 15 mph too high - max 346, measured 436 (@25mph max 523, meas 411)

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Trying to figure out what the reasons for NOX value being over max could be. All other values are within norms and would make for a passing smog check. When starting the engine I get some times hesitation and was told that at 15 mph there's a vibration/hesitation going on... any ideas?

My car has 206000 miles.
My car has an automatic transmission.

Hi there. NOx is produced through heat; which typically occurs when the engines fuel mixture is too lean. Lean fuel mixtures cause high NOx or nitric oxide emissions. A lean fuel condition may be due to a vacuum leak’s and/or defective fuel control components, such as the Air Flow Meter, Engine Coolant Temperature Sensor, and O2 sensors.

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