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Forum > Failed Smog Test > Question

1998 Volkswagen GTI failed NOx Emissions. Why?

Hi I was curious on what would cause my VW car to fail the NO portion of the smog test? All the other numbers for HC and CO are good but it failed the NOX portion at 25 mph test but passed the 15 mph test.
Answer:

Your Volkswagen is a non-EGR equipped vehicle (it doesn't have an EGR valve or EGR valve system which keeps NOx low). The lowering of NOx is handled by your VW's Catalytic Converter (CAT) and fuel control system. If the CAT is bad or the engine is running too lean (very low CO), this could cause high NOx. We'd recommend having a smog check repair center conduct a fuel feedback test. This would ensure your Volkswagen is in proper air/fuel control, and if no problems are found there, suspect a defective CAT. posted by SmogTips Support

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