Why has my mileage dropped from 410 miles a tank to 230 miles a tank? on 2007 Kia Rio

My check engine light is on. I know that is a bad oxygen sensor. I think the cataletic converter is failing. That is all I can think of that would affect mileage so drastically without affecting performance other that the engine is a little louder now. When it first started mileage went from 400 miles a tank to 300. Now it is at 230 miles a tank. Opinion?

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