Why is my Hyundai Elantra failing emissions tests o r showing high NOx levels?

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Why is my Hyundai Elantra failing emissions tests o r showing high NOx levels?

Some Hyundai Elantra owners, particularly those with older models o r high mileage, report failing emissions inspections due to elevated nitrogen oxide NOx levels. This may be caused by a failing catalytic converter, malfunctioning oxygen sensors, o r incorrect air-fuel mixtures. What are the most likely culprits, and how can these be diagnosed before an emissions test?

Gari Specialist asked on 21 Jun 2025 18:04:18 pm

1 Answer

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Gari Master - on 21 Jun 2025 18:07:48 pm

High levels of NOx nitrogen oxides during emissions testing typically indicate that your engine is running too hot o r lean. This is more common in older o r high-mileage Elantras.
Possible causes include:

Worn o r failing catalytic converter

Malfunctioning oxygen O2 sensors

EGR valve malfunction if equipped

Improper air-fuel mixture
Solution:
Use an OBD-II scanner to check for trouble codes. Replacing faulty O2 sensors and ensuring the catalytic converter is functioning correctly can resolve most issues. A professional emissions diagnostic may be required if there are no active codes but emissions levels are high.