What could have happened to my car?

What is your question?

Few days ago I tried to drive my car and it did not go more than 5 mph. The engine was running but the car barely moved. Took it to the dealer where I was told it was the oxygen sensor, they replace it, but it did not resolve the issue. Now they are telling me I need to pay 3.400 $ for a new turbo catalyst converter. any advise?