check engine stays on on 1996 Volkswagen Passat

oxygen sensor replaced and was sent to vw dealer for connection

Asked by for the 1996 Volkswagen Passat
3 answers
What codes do you have now?
why did you have to do that?
My "check engine light" keeps going on and off. German Automotive repair shop said it needs the entire catalytic converter replaced. But it seems to run fine. And the light is now off. WTF?