Emission Control problems of the 1996 Volkswagen Cabriolet

From the time I bought the car the gas mileage was grossly low so everytime I took it in for scheduled service I would have them check out the fuel system nothing was ever done and the gas mileage has never gotten better now I have approx. 42,000mi on the vehicle and still horrible gas mileage and no power ii thought it was catalytic converter which is covered under the fed emmissions warranty but when I took it to the dealer they said it was the oxygen sensors which are connected to the catalytic converter I have done some research on these O2 sensors and everything I have read leads me to believe that they were faulty from time of purchase!.