Comment by buran77

On the other hand the quality and performance of those $80 catalytic converters are questionable at best. They have neither the longevity, nor the performance of the original part. They might last even 10 times less, and they're usually just barely good enough to pass the emissions tests, which is already the lowest bar to pass given how all manufacturers optimize for that. Real life emissions are far worse.

And the purpose of the catalytic converter is to make sure the CO, NOx, and unburned fuel are rapidly oxidized to CO2, N, and water before leaving the exhaust system. The outcome is that you will produce more greenhouse gases but fewer compounds that are more immediately dangerous to people, especially in cities. So it reduces localized pollution at the price of more CO2.