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2001 Hyundai Accent failed smog test for high NOx and high HC.

Hi. 2001 Hyundai Accent 1.5L has high NO and HC. i need to find out what causes high NOx (1500 ppm) and slightly high HC (68 ppm)? Also, Check Engine Light for "intake too lean" comes and goes by itself once in a while. However, the car runs really good and gets unusually high fuel economy, 40+ mpg (because of too lean condition). People tell me to replace MAF sensor, O2 sensor, Catalytic converter etc. What do you think is the most probable culprit?
Answer:

The answer lies within the CO numbers. If in fact your Hyundai is running lean we'd recommend first conducting a fuel "feedback" test to ensure the oxygen sensor is responsive and functioning, and then suspect the Mass Air Flow (MAF) sensor. The CAT has no control on fuel delivery and will not be responsible for the lean fuel condition. It's primary function is to reduce HC, CO, and NOx. If all checks well with fuel control (MAF, O2, ECT), you may conduct an "intrusive" CAT test (this is where they make a small hole and take a smog sample from before and after the CAT) to find out your Catalytic Converter operation. posted by SmogTips Support

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